KANDAHAR, Afghanistan - A roadside bomb yesterday killed four US Marines sent to southwestern Afghanistan to help train the country's fledgling police.
It was the deadliest attack on American forces in Afghanistan this year.
The explosion occurred a day after Defense Secretary Robert Gates highlighted the fact that more American and allied troops were killed in Afghanistan than in Iraq last month.
Meanwhile, US and NATO troops helped Afghan forces search for 870 inmates who escaped from Kandahar's main prison on Friday after a sophisticated Taliban assault that even NATO conceded was a success for the militants.
The Marines attacked in the roadside bombing in Farah Province were from the Second Battalion, Seventh Regiment based in Twentynine Palms, Calif.
They arrived in Afghanistan earlier this year and were sent to southern and western Afghanistan to train police.
Another Marine was wounded in the blast.
Gates told his counterparts meeting in Brussels recently 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 US troops killed in Afghanistan this year, according to an Associated Press count.
No more than two US personnel had been killed in any one attack in Afghanistan this year.
Afghanistan's deputy interior minister, Munid Mangal, said about 1,000 prisoners, including criminals and hundreds of Taliban insurgents, were housed in Kandahar's Sarposa Prison when dozens of militants on motorbikes attacked the facility late Friday. Seven police and several prisoners died in the assault, he said.
One suicide bomber detonated a tanker truck full of explosives at the prison gate while a second bomber blasted another escape route through a back wall. Rockets fired from inside the prison's courtyard collapsed an upper floor.
The police chief of Kandahar Province, Sayed Agha Saqib, said 390 Taliban prisoners were among the 870 inmates who escaped.
Brigadier General Carlos Branco, the chief spokesman for NATO's International Security Assistance Force, conceded that the militants pulled off a "very successful operation."
"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."
NATO was providing intelligence, surveillance and reconnaissance assets to help track fleeing militants, Branco said.
US forces helped transport Afghan Army personnel to the scene "so that they could catch the prisoners who escaped," said US Captain Christian Patterson.
There were no indications that the militants received help from the inside, but the prison's chief official, Abdul Qadir, was placed under investigation for possible involvement, said Mohammad Qasim Hashimzai, a deputy minister at the Justice Ministry.
Afghan officials warned that the Taliban essentially boosted its force by 400 fighters - including several thwarted suicide bombers - because of the prison break, but Branco said NATO officials didn't think it would change the military situation.
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.
"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 the past two years between insurgents and NATO troops, primarily from Canada and the United States.