Four American soldiers were killed by a roadside bomb Saturday in Afghanistan, the U.S. military said.
Lt. Col. David Johnson said the blast targeted a vehicle being used by U.S. personnel to help train the Afghan police. One U.S. soldier was also seriously wounded in the attack in western Farah province.
It was the deadliest attack against U.S. troops in the country this year, officials said.
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.
However, Johnson said he could not immediately confirm that the four personnel killed were Marines.
The bombing comes a day after 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 had 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.