JALALABAD, Afghanistan — A U.S. soldier was killed in eastern Afghanistan Thursday when an Afghan policeman opened fire on an American patrol returning to its base.

The Afghan National Policemen opened fire and threw a hand grenade at the U.S. patrol as it returned from a meeting with tribal elders in Bermel District, Paktika Province, near the border with Pakistan. The remaining U.S. troops killed the policeman, according to the military.

The military has launched an investigation into the incident.

In a separate incident, another coalition soldier was killed by possible friendly mortar fire in northeastern Afghanistan, the military said.

Neither of the soldiers killed Thursday have been identified pending notification of their next of kin.

Most U.S. bases in Afghanistan include sizable contingents of Afghan troops, and American units rarely patrol without including the Afghans. But Thursday’s incident was the second time in less than a month that an Afghan officer has killed a U.S. soldier, raising concerns that militants may have infiltrated the Afghan police force.

Last month, an Afghan policeman opened fire on U.S. troops at a police station in Paktia province, killing an American soldier and wounding three other troops. U.S. forces then killed the policeman.

Two U.S. troops were killed in May 2007 when a soldier with the Afghan National Army fired into their vehicle following a visit to a prison near Kabul. The soldier was killed by fellow Afghan troops, and a spokesman for the Afghan Ministry of Defense later said the soldier had twice been treated for mental illness.

Thursday’s incident in northeastern Afghanistan began when a patrol of coalition and Afghan soldiers came under small-arms fire in a village. The soldiers called for indirect fire and may have been hit by an errant mortar round, according to a press release from United States Forces-Afghanistan.

Several coalition and Afghan soldiers who were hurt in the incident clearly appeared to be suffering from shrapnel wounds.

U.S. medics who were dropped in by helicopter to evacuate the wounded continued to come under heavy fire from the surrounding mountains, and two U.S. medevac helicopters were hit by small-arms fire. Neither was seriously damaged.

Army Staff Sgt. Bradley Robbins, one of the medics on the ground, said an Afghan soldier lost an arm and a leg and another soldier received a serious shrapnel wound to chest.

"We took several incoming rounds into the area where I was at," Robbins said. "They were hitting right behind me. And then the aircraft got hit. But we got them out of there."