About 870 prisoners escaped during a Taliban bomb and rocket attack on the main prison in southern Afghanistan that knocked down the front gate and demolished a prison floor, Afghan officials said Saturday. And in western Afghanistan on Saturday, a roadside bomb exploded near a U.S. military vehicle, killing four Americans in the deadliest attack against U.S. troops in the country this year, officials said.

    The bomb in the western province of Farah targeted Marines helping to train Afghanistan's fledgling police force, said U.S. spokesman Lt. Col. David Johnson. One other Marine was wounded in the attack.

    Marines from the 2nd Battalion, 7th Regiment based in Twentynine Palms, California, arrived in Afghanistan earlier this year and were sent to southern and western Afghanistan to train police.

    The bombing comes one day after U.S. Defense Secretary Robert Gates told his counterparts in Europe that for the first time, the monthly total of American and allied combat deaths in Afghanistan exceeded the toll in Iraq during May.

    The four deaths bring to at least 44 the number of U.S. troops killed in Afghanistan this year, according to an Associated Press count. No more than two U.S. personnel had been killed in any one attack in Afghanistan this year, according to the AP tally.

    In the prison escape, the police chief of Kandahar province, Sayed Agha Saqib, said 390 Taliban inmates were among those who fled the prison during the attack late Friday.

    NATO's International Security Assistance Force put the number of escapees slightly higher, at around 1,100, according to spokesman Brig. Gen. Carlos Branco. He conceded that the assault was a success.

    "We admit it," Branco said. "Their guys did the job properly in that sense, but it does not have a strategic impact. We should not draw any conclusion about the deterioration of the military operations in the area. We should not draw any conclusion about the strength of the Taliban."

    The complex attack included a truck bombing at the main gate, a suicide bomber who struck a back wall and rockets fired from inside the prison courtyard, setting off a series of explosions that rattled Kandahar, the country's second biggest city.

    The rockets demolished an upper prison floor, said Mohammad Qasim Hashimzai, a deputy minister at the Justice Ministry. Nine police were killed in the attack, said Interior Ministry spokesman Zemeri Bashary.

    There were no indications that the militants received help from the inside, but as a precaution the prison's chief official, Abdul Qabir, was placed under investigation for possible involvement, Hashimzai said.

    A Taliban spokesman, Qari Yousef Ahmadi, said 30 insurgents on motorbikes and two suicide bombers attacked Sarposa Prison.

    NATO was providing intelligence, surveillance and reconnaissance assets to help track fleeing militants, Branco said.

    Afghan officials warned that the Taliban essentially boosted its force by 400 fighters because of the prison break, but Branco said NATO officials didn't think it would change the military situation.

    "OK, they got some more fighters, more shooters," Branco said. "These guys who escaped from the prison are not going to change the operational tempo and they do not provide the Taliban with operational initiative."

    A man who claimed to be one of the militants who escaped, Abdul Nafai, called an Associated Press reporter and said the insurgents had minibuses waiting outside the prison during the attack and that dozens of militants fled in the vehicles. Other witnesses and officials said the militants fled on foot into pomegranate and grape groves behind the prison.

    Hashimzai said the jail did not meet international minimum standards for a prison. The Kandahar facility was not built as a prison but had been modified into one, he said.

    A delegation of deputy ministers from the Justice and Interior ministries left for Kandahar early Saturday.

    "Plans are under way to renovate all the prisons around the country," said Hashimzai. "Kandahar was one of them, but unfortunately what happened last night is cause for concern."

    Kandahar was the Taliban's former stronghold and its province has been the scene of fierce fighting in the past two years between insurgents and NATO troops, primarily from Canada and the United States.

    Qabir, the chief of Kandahar's Sarposa Prison, said the assault began when a tanker truck full of explosives detonated at the prison's main entrance, wrecking the gate and a police post, killing all the officers inside.

    Soon after, a suicide bomber on foot blasted a hole in the back of the prison, Qabir said.

    Ahmadi, the Taliban spokesman, said militants had been planning the assault for two months.

    Canadian soldiers with NATO's International Security Assistance Force helped provide a security cordon after the attack.

    Last month, some 200 Taliban suspects at the prison ended a weeklong hunger strike after a parliamentary delegation promised that their cases would be reviewed.

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     Associated Press Writer Jason Straziuso contributed to this report from Kabul.