London - President Obama and Russian President Dimitri Medvedev launched negotiations on a new nuclear arms treaty today, even as they agreed to pursue new and broader cooperation across a wide range of policy areas.

    In a statement after a closed-door meeting, the two leaders pledged to begin immediately on a new treaty on offensive nuclear weapons to replace the START agreement, which expires at the end of this year.

    "The Presidents decided to begin bilateral intergovernmental negotiations to work out a new, comprehensive, legally binding agreement on reducing and limiting strategic offensive arms to replace the START Treaty," the statement says. "The United States and the Russian Federation intend to conclude this agreement before the Treaty expires in December."

    The agreement commits the two nations to reduce their nuclear arsenals to levels lower that those mandated by the 2002 Moscow Treaty on Strategic Offensive Reductions, which call for both nations to have between 1,700 and 2,200 warheads by December 31, 2012.

    Obama told reporters that he will travel to Moscow in July, the date by which the two leaders said their negotiators should report progress on the new arms reduction treaty.

    The two men - both in London for the upcoming G-20 summit -also issued a broader statement that outlines new areas of planned cooperation while skirting some of the most contentious issues that have soured relations during the past several years.

    They pledged to cooperate on trying to rein in Iran's nuclear ambitions, contain the proliferation of nuclear technology, and fight terrorism in Afghanistan and Pakistan. And they expressed joint concern about North Korea's expected launch of a ballistic missile sometime soon.

    "We, the leaders of Russia and the United States, are ready to move beyond Cold War mentalities and chart a fresh start in relations between our two countries," the statement reads. "In just a few months we have worked hard to establish a new tone in our relations. Now it is time to get down to business and translate our warm words into actual achievements of benefit to Russia, the United States, and all those around the world interested in peace and prosperity."

    The statement reflects a desire by both nations to put more pressure on Iran to end its pursuit of nuclear weapons. While giving a nod toward Iran's claim that it is developing a civilian nuclear program, the document puts the Russians on the record calling on Iran to implement U.N. Security Council resolutions regarding the confirmation of its nuclear capabilities and intentions.

    "Iran needs to restore confidence in its exclusively peaceful nature," the document says.

    The statement does not attempt to resolve some of the stickiest issues that divide Washington and Moscow, in particular the disagreement over Russia's aggressive actions in the nation of Georgia and the deployment of missile defense equipment in Poland.

    On Georgia, the statement says: "Although we disagree about the causes and sequence of the military actions of last August, we agreed that we must continue efforts toward a peaceful and lasting solution to the unstable situation today."

    It acknowledges that "differences remain over the purposes of deployment of missile defense assets in Europe," but adds that the two leaders discussed the possibility of international cooperation in the missile defense field.

    Wednesday's meeting was the first ever for Obama and Medvedev. Earlier in the day, Obama sat down with British Prime Minister Gordon Brown to formally open his first overseas trip. Obama and Brown then held a joint news conference that focused mostly on the global economic collapse that world leaders have gathered here to fix.

    Obama told reporters that America accepts its share of the blame for crisis, but said recovery will depend on the world's nations acting together to spark growth and revamp economic regulations.

    The United States "has some accounting to do," the president said, because of a financial regulatory system that proved inadequate to oversee the growth of banks and hedge funds.

    But he added that there has been an equal "mismatch between regulatory reforms and the highly integrated global markets" and said he expects a concerted approach to be announced at the economic summit which begins with a working dinner Wednesday night.

    "The United States does not intend to act alone, and we are not," Obama said after a closed-door meeting with Brown.

    "The world has become accustomed to the United States being a voracious consumer market and the engine that drives a lot of economic growth worldwide," Obama said. "If there's going to be renewed growth, it can't just be the United States as the engine. Everybody is going to have to pick up the pace."

    Both Obama and Brown dismissed talk of dissension among the governments gathered here about how best to proceed with economic recovery and whether to spend more money to spark growth. Obama called reports of those disagreements "vastly overstated."

    To reports that French President Nicholas Sarkozy might pull out of the summit if the negotiations don't go his way, Brown joked that he was "confident that he will still be sitting as we complete our dinner this evening."

    In advance of his meeting with Medvedev, Obama said he has no interest in "papering over" the "very real differences between the United States and Russia."

    But he said that there are "broad interests" that can form the basis of a cooperative relationship, and that he detects "great potential" for working together on a broad range of topics.

    "Both the United States and Russia and other nuclear powers will be in a much stronger position to strengthen what has become a somewhat fragile, threadbare non-proliferation treaty if we are leading by example," Obama said.

    Obama acknowledged that the threat from nuclear warfare has receded since the end of the Cold War, but called the threat of terrorists or others getting their hands on the weapons the "greatest threat to humanity."

    As Obama began his string of meetings - in addition to Gordon and Medvedev, Wednesday's lineup includes Chinese premier Hu Jintau, and British opposition Leader David Cameron - thousands of protesters lined the streets in anticipation of the G-20 summit, which officially begins tonight.

    Several of the protests are taking their name from the April Fools Day holiday. A "Financial Fools Day" protest is expected to draw large crowds of demonstrators marching from several central London train stations to the Bank of England headquarters.

    Police in bright yellow vests massed as the crowds swelled in London's streets, repelling surges of people towards them and working to maintain order.

    Environmental campaigners are staging a "Fossil Fools Day" demonstration near the ExCeL center, where the G20 meetings are taking place. Large protests are also planned by the Stop the War Coalition, the Campaign for Nuclear Disarmament, the Palestine Solidarity Campaign and other groups.

    Many of the protests will be held around "The City," as London's financial district is known. Banks have taken special precautions to protect their employees amid fears that bankers could be targeted by protesters.

    On a lighter note, Obama declined during the news conference to be drawn into predictions about the outcome of soccer games in Europe, citing controversy in the United States over his NCAA tournament bracket.

    He said the only campaign advice he would give Brown, who is running for re-election, was that "over time, good policy is good politics."

    The new U.S. president was also careful to meet the protocol requirements that go with carefully scripted overseas trips.

    In the first moments at the microphone, Obama mentioned the "special relationship" that the United States has with Britain, a pet phrase that his spokesman failed to use several weeks ago, prompting mocking headlines in the British tabloid press.

    Obama also made certain to articulate his anticipation about the audience he and first lady Michelle Obama will have with Queen Elizabeth II later Wednesday.

    "As you might imagine, Michele has really been thinking that through," Obama said. "In the imagination of people throughout America, what the queen stands for and her decency and her civility and what she stands for, that's very important."

    Michelle Obama spent Wednesday morning with Sarah Brown, the prime minister's wife, touring a facility for cancer patients and their families before attending a series of closed-door teas.

    The two women visited Maggie's Center, which has provided support for 12,000 cancer families in the year it has been opened.

    They visited with three male cancer patients and then attended a "Look Good, Feel Better" make-up session with two female patients.

    "This is pretty amazing. It's an oasis. An oasis that is necessary for people who are struggling," Obama said as she sipped tea with Brown a bit later. "It's a quiet place that makes people feel whole."

    Mrs. Obama was dressed in a bright green pencil skirt, a cream-colored, sequined cardigan, very high heels and pearls. She was expected to change her clothes, aides said, before her meeting with the queen later Wednesday afternoon.

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    Correspondent Mary Jordan contributed to this report.