Moscow - President Dmitri Medvedev of Russia on Thursday said that Russia would act as an international guarantor of South Ossetia and Abkhazia, the two pro-Russian enclaves at the center of the crisis that have long desired separation from Georgia.

    As Secretary of State Condoleezza Rice headed to the region for discussions on the crisis and to show support for Georgian President Mikheil Saakashvili, the Russian position seemed to be a direct challenge to President Bush who said a day earlier that he "insists that the sovereign and territorial integrity of Georgia be respected."

    Meanwhile in Georgia, Russian forces briefly allowed the Georgian police to return to the city of Gori on Thursday morning as the Russian troops appeared to be preparing to pull out. But joint patrols were canceled three hours later and the city returned to Russian control. In Poti, a Black Sea port further west, Russian tanks patrolled the city, a sign that Russian forces remained in control of key parts of Georgian territory.

    Mr. Medvedev said he would support the independence aspirations of South Ossetians and Abkhazians if they were in accordance with the United Nations Charter, international conventions of 1966 and the Helsinki Act on Security and Cooperation in European.

    "You have been defending your land and the right is on your side," Mr. Medvedev said, at a meeting with leaders of the two breakaway regions.

    As she traveled to the region, Ms. Rice was due to arrive first in France to meet the French president, Nicolas Sarkozy, who brokered the cease-fire between Russia and Georgia, in the president's summer residence in southern France, at 3 p.m., local time, Mr. Sarkozy's office said. She would later travel to Tbilisi, the Georgian capital.

    On Wednesday, the United States and Georgia called the Russian advances into Gori and another strategic Georgian city a violation of the cease-fire agreement struck only hours earlier.

    In response, Mr. Bush sent American troops to Georgia to oversee a "vigorous and ongoing" humanitarian mission, in a direct challenge to Russia's display of military dominance over the region. Mr. Bush demanded that Russia abide by the cease-fire and withdraw its forces or risk its place in "the diplomatic, political, economic and security structures of the 21st century." It was his strongest warning yet of potential retaliation against Russia over the conflict.

    In Gori, which was the focus of international protest after Russia shelled it and occupied it on Wednesday, the joint patrols on Thursday suggested a cooling of tensions in the city.

    Gori is just 40 miles from the Georgian capital, Tbilisi, and rumors had circulated on Wednesday of a possible advance on the capital.

    It was not clear why the joint patrols failed, but it appeared that personnel on the ground were in conflict. Around 10 a.m. Thursday, a Russian Army major gave orders to Georgian and Russian police officers to patrol in pairs. But this clearly did not last. "We had to go or there would have been shooting," said a Georgian officer, who would not give his name.

    More than 30 Georgian police left Gori and returned to a Georgian post outside the city and shortly afterward Russian troops fired three artillery rounds. Their target was not clear.

    In Poti, three Russian tanks were sighted patrolling the city. Villagers there said the Russian tanks frequently made the 30-minute drive from their base in Senaki to Poti to perform exercises on an abandoned military base, with troops jumping off their tanks and sweeping the area around them.

    In an interview on Ekho Moskvy, Sergei Lavrov, Russia's foreign minister, said Georgia's territorial integrity was "de facto limited because of the war," and said any agreement suggesting otherwise would be "deeply insulting" to the people of South Ossetia and Abkhazia.

    He also said he was not worried about the threat of international isolation, Interfax reported.

    "I don't know how they are going to isolate us," he said.

    The decision Wednesday to send the American military, even on a humanitarian mission, deepened the United States' commitment to Georgia and America's allies in the former Soviet sphere, just as Russia has been determined to reassert its control in the area.

    On a day the White House evoked emotional memories of the cold war, a senior Pentagon official said Wednesday that the relief effort was intended "to show to Russia that we can come to the aid of a European ally, and that we can do it at will, whenever and wherever we want." At a minimum, American forces in Georgia will test Russia's pledge to allow relief supplies into the country; they could also deter further Russian attacks, though at the risk of a potential military confrontation.

    "We expect Russia to ensure that all lines of communication and transport, including seaports, airports, roads and airspace, remain open for the delivery of humanitarian assistance and for civilian transit," Mr. Bush said. "We expect Russia to meet its commitment to cease all military activities in Georgia, and we expect all Russian forces that entered Georgia in recent days to withdraw from that country."President Mikheil Saakashvili of Georgia, who has sharply criticized what he called a failure of the West to support his country, declared the U.S. relief operation a "turning point" in the conflict, which began last Thursday when Georgian forces tried to establish control in the breakaway region of South Ossetia, only to be routed by the Russians.

    "We were unhappy with the initial actions of the American officials, because they were perceived by the Russians as green lines, basically, but this one was very strong," he said in a telephone interview after Mr. Bush's statement in Washington.

    Mr. Saakashvili interpreted the aid operation as a decision to defend Georgia's ports and airports, though Bush administration and Pentagon officials quickly made it clear that would not be the case. A senior administration official said, "We won't be protecting the airport or seaport, but we'll certainly protect our assets if we need to."

    Mr. Bush spoke in the Rose Garden of the White House, flanked by his secretaries of state and defense, Condoleezza Rice and Robert M. Gates. He said that Ms. Rice would fly to France to support its mediation efforts and then to Georgia "to continue our efforts to rally the free world in the defense of a free Georgia."

    State Department officials said there were no plans for Ms. Rice to go to Moscow. Mr. Bush's remarks, like the military operation he ordered, reflected a growing apprehension within the White House over Russia's offensive, as well as mounting frustration that Prime Minister Vladimir V. Putin of Russia, whom Mr. Bush often calls a friend, was unmoved by appeals for moderation. Underscoring the urgency, Mr. Bush, who had remained at the Olympics in Beijing while the conflict erupted, postponed a planned trip to his ranch in Crawford, Tex., which was to have begun on Thursday.

    The first relief aircraft, a C-17 transporter carrying medical supplies and materials for shelter for thousands displaced by the fighting, arrived in Tbilisi, Georgia's capital, on Wednesday; a second was due Thursday.

    Ms. Rice called her Russian counterpart, Sergey V. Lavrov, and informed him about the relief operation. The presence of American troops to help the aid mission will also allow the United States to monitor whether Russia was honoring the cease-fire, brokered by President Nicolas Sarkozy of France.

    At a news conference at the State Department, Ms. Rice evoked some of the darkest memories of the cold war, though she stopped well short of promises of direct military support to Georgia.

    "This is not 1968, and the invasion of Czechoslovakia, where Russia can invade its neighbor, occupy a capital, overthrow a government and get away with it," she said. "Things have changed."

    She and Mr. Bush gave credence to Georgia's accusations that Russian forces continued to operate in violation of the cease-fire. Russia insisted that all of its operations were permitted under the agreement.

    The cease-fire included a provision that required Russian forces to withdraw to their "normal bases of encampment" but also allowed them to "implement additional security measures."

    A senior American official said the vague language "would allow the Russians to do almost anything."

    Only hours after the agreement was reached early Wednesday, a Russian tank battalion occupied parts of Gori, a strategic city in central Georgia. Hundreds of additional Russian soldiers also poured over the border from Russia into South Ossetia, accompanied by fuel trucks and attack helicopters.

    The presence of Russian forces in Gori frayed nerves as rumors circulated of an attack on Tbilisi itself. A Russian battalion commander, at a checkpoint on the highway from Gori to the capital, spoke menacingly of Mr. Saakashvili.

    "If he doesn't understand the situation, we'll have to go further," the commander said on the condition of anonymity. "He doesn't seem to understand that the Russian Army is much stronger than the Georgian Army. His tanks remain in their places. His air force is dead. His navy is also. His army is demoralized."

    Mr. Bush also cited reports that Russians had taken up positions in Poti, a port city on the Black Sea, and were blowing up Georgian ships. Russian officials denied that troops had occupied any cities, but some of the statements appeared to rest on technicalities of what constituted occupation.In Russia, Mr. Lavrov, the foreign minister, warned the Bush administration that it risked a breach with Russia by throwing its support so strongly behind Georgia and its president.

    "We understand that this current Georgian leadership is a special project of the United States," he said, "but one day the United States will have to choose between defending its prestige over a virtual project or real partnership" with Russia.

    He and other officials said that Russian troops in Georgia continued to conduct operations on Wednesday, but only in support of peacekeepers, a role explicitly permitted in the cease-fire signed by Russia's president, Dmitri A. Medvedev, and Mr. Saakashvili.

    Mr. Lavrov said Wednesday that when Georgian forces abandoned their military headquarters near Gori, they left "a major arsenal of armaments and military equipment" and that the Russian troops were now guarding it.

    "To leave it in such a condition would be unforgivable," he said. "The city residents have problems with food," he added. "The Russian servicemen will provide them with necessary aid."

    One of the Russian commanders, Gen. Vyacheslav Nikolayevich of the Pskov Airborne Division, said on Wednesday that Russian soldiers would remain on the outskirts of Gori but not enter the city. "People can get back to their lives," he said.

    Asked whether Mr. Bush's relief mission made him nervous, he scoffed. "What can the Americans do to us?" he said. "A big country like Russia doesn't fear America."

    Mr. Bush's remarks were the toughest yet in the conflict. "Russia's ongoing actions raise serious questions about its intentions in Georgia and the region," he said. "In recent years, Russia has sought to integrate into the diplomatic, political, economic, and security structures of the 21st century. The United States has supported those efforts. Now Russia is putting its aspirations at risk by taking actions in Georgia that are inconsistent with the principles of those institutions."

    Administration officials said that the United States would not take part in planned military exercises with Russia this weekend and that they were considering blocking Russia's accession into the World Trade Organization and its participation in the Group of 8 industrialized nations.

    "I don't think that there's any doubt that Russia has already in its actions called into question some of its desire to be integrated into these institutions," Ms. Rice said Wednesday.

    In South Ossetia, investigators began to look into accusations of atrocities. Human Rights Watch reported that researchers witnessed "terrifying scenes of destruction" in four ethnic Georgian villages, and said the villages had been looted and burned by South Ossetian militias.

    Anna Neistat, one of the researchers, said by telephone from Tskhinvali, the capital of South Ossetia, that they had found no evidence to substantiate Russian assertions of widespread brutality by Georgian troops. Human Rights Watch has been able to confirm fewer than 100 deaths.

    Russian leaders have said they would like to bring Mr. Saakashvili to face war crimes charges in The Hague. Meanwhile, Georgia has filed a lawsuit against Russia at the International Court of Justice in The Hague for its actions on and around Georgia from 1991 to 2008, the court said in a statement.

    At the United Nations, Security Council members continued informal but inconclusive consultations on the final draft of a resolution intended to codify the cease-fire. Foreign ministers from 27 European Union countries convened an emergency meeting in Brussels.

    Wednesday was a day on which Georgians were teased with signs that the Russians were - or were not - coming. There were a flurry of reports that Russian tanks were on the road from Gori to Tbilisi, but no tanks ever arrived there.

    In Senaki, Russian soldiers had occupied Lia Baramia's cafe. She had fled when she heard about the fighting. When she returned, Russian soldiers had dug trenches in the cafe's driveway and were using an outdoor tap to drink and bathe. They were friendly, she said, but she was happy when they left. Cows were munching on the leaves and grass the soldiers had used to camouflage their vehicles, and Ms. Baramia decided to reopen.

    Within 10 minutes, a convoy of Russian personnel carriers sped back into town.

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    C.J. Chivers reported from Gori, Georgia, and Steven Lee Myers reported from Washington. Sabrina Tavernise also reported from Gori, and Ellen Barry from Moscow. Reporting was contributed by Thom Shanker and Helene Cooper from Washington; Dan Schneider from the United Nations; and Andrew Kramer from Tbilisi, Georgia.