On Sunday, U.S. officials confirmed the deaths of two Americans, a service member and a civilian contractor killed late Saturday.
The fighting started when insurgents attacked a checkpoint set up by U.S. forces in eastern Wardak province, said Shahidullah Shahid, a provincial government spokesman. He said the insurgents apparently used mortars in the attack. The Americans thought they were under attack from their allies at a nearby Afghan army checkpoint and fired on it. The Afghan soldiers returned fire, Shahid said.
The Afghan Defense Ministry spokesman said the shooting broke out as a result of a "misunderstanding" while ISAF forces were on patrol near an Afghan army checkpoint.
NATO's International Security Assistance Force, commonly referred to as ISAF, gave a different account of the fighting in Sayd Abad district.
"After a short conversation took place between (Afghan army) and ISAF personnel, firing occurred which resulted in the fatal wounding of an ISAF soldier and the death of his civilian colleague," the coalition said in a statement. It said the three Afghan soldiers died "in an ensuing exchange of fire."
NATO did not say whether it considered this an "insider" attack on foreign forces by Afghan allies.
In Washington, Pentagon press secretary George Little said 2,000 deaths is one of the "arbitrary milestones defined by others " that the U.S. administration does not mark.
"We honor all courageous Americans who have paid the ultimate sacrifice in Afghanistan to make the American people more secure," he said. "The fact of the matter is that America is safer because of all of those who have served in this war, including our fallen heroes."
In addition to the 2,000 Americans killed since the Afghan war began on Oct. 7, 2001, at least 1,190 more coalition troops from other countries have also died, according to iCasualties.org, an independent organization that tracks the deaths.
According to the Afghanistan index kept by Brookings, about 40 percent of the American deaths were caused by improvised explosive devices. The majority of those were after 2009, when President Barack Obama ordered a surge that sent in 33,000 additional troops to combat heightened Taliban activity. The surge brought the total number of American troops to 101,000, the peak for the entire war.
According to Brookings, hostile fire was the second most common cause of death, accounting for nearly 31 percent of Americans killed.
Tracking deaths of Afghan civilians is much more difficult. According to the U.N., 13,431 civilians were killed in the Afghan conflict between 2007, when the U.N. began keeping statistics, and the end of August. Going back to the U.S.-led invasion in 2001, most estimates put the number of Afghan civilian deaths in the war at more than 20,000.