(CNN) -- Six American forces serving under NATO's International Security Assistance Force were killed in an insurgent ambush while patrolling in eastern Afghanistan, a NATO spokesman told CNN on Saturday.
A U.S. soldier aboard a helicopter patrols an area north of Kabul, Afghanistan, last month.
Friday's deaths make this year the deadliest since the 2001 invasion for U.S. military personnel. In addition, eight other NATO soldiers were wounded. It was not immediately clear whether those were Americans.
The U.S. troops were on joint-foot patrol with Afghan National Army soldiers -- three of whom were killed and 11 who were wounded, NATO said.
The combined forces came under fire from small arms and rocket-propelled grenades during the "ambush."
In response, NATO said forces fired back with small-arms, machine guns, mortars, artillery and close-air support.
At least one insurgent was killed in the clashes.
So far this year 100 U.S. military personnel have died due mainly to insurgency-related violence, according to a CNN count based on figures from the U.S.-led NATO and Coalition. That number surpasses the previous high of 94 U.S. military deaths in 2005. E-mail to a friend