A roadside bomb exploded near a U.S. military vehicle on Saturday, killing four Americans in western Afghanistan in the deadliest attack against U.S. troops in the country this year, officials said.

The bomb in the western province of Farah targeted U.S. personnel helping to train Afghanistan's fledgling police force, said U.S. spokesman Lt. Col. David Johnson. One other American 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. However, Johnson said he could not immediately confirm that the four personnel killed were Marines.

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.