US and Afghan troops have come under direct fire in eastern Afghanistan, and there are reports of casualties.
US officials say it is not clear whether militants in Afghan uniforms attacked the US forces in Nangarhar province or whether there was an argument between the allies.
An investigation is now under way.
Some 13,000 US troops remain in Afghanistan after the US intervention to oust the Taliban following the 11 September 2001 attacks.
Several US service personnel were injured or killed in Saturday's incident, a US official was quoted by the Associated Press as saying.
Meanwhile, two Afghan officials said five or six US soldiers and six Afghan soldiers were killed, the New York Times reports.
