US military death toll in Afghanistan reaches 2,000
30 September 2012 Last updated at 15:34 GMT
A US soldier and contractor were killed while three Afghan soldiers died and several were injured.
The new deaths occurred on Saturday in Wardak province.
The international mission, Isaf, initially said the soldier was believed to have been killed by a member of the Afghan security services, but it later said the circumstances were unclear.
What is known is that a firefight took place, after what Isaf described as a short conversation between coalition and Afghan soldiers, says the BBC's Quentin Sommerville in Kabul.
Isaf says "insurgent fire" may have been involved in the incident, which is now under investigation by a joint Afghan and coalition team, adds our correspondent.
The American death toll goes back to the US-led invasion of Afghanistan in 2001.