Afghan officials say an Afghan security officer opened fire on U.S. troops while conducting a joint operation with them, killing two and wounding two others.
A spokesman for the governor of Wardak province said the shooting occurred late Friday while Afghan and U.S. forces were on patrol together in Wardak.
It is unclear what motivated the shooting.
A statement from NATO-led forces in Afghanistan said two U.S. soldiers died from injuries sustained in a "hostile attack" Friday, but declined to give further details.
NATO says two other U.S. troops died Friday in a bomb attack in southern Afghanistan. Another died of wounds sustained in a bomb attack in eastern Afghanistan.
U.S. and NATO troops in Afghanistan have recently been facing stiffer Taliban resistance.
2009 is the deadliest year for foreign forces since the conflict began nearly eight years ago.
Some information for this report was provided by AP and Reuters.
An Afghan security officer opened fire on American troops while conducting a joint operation with them, Afghanistan officials reported. Two soldiers were killed and two others were injured.

A spokesman for the governor of Wardak province said the shooting happened late on Friday, while both countries' forces were patrolling the area together. It was not immediately clear what motivated the shooting.

NATO-led forces in Afghanistan released a statement saying two US soldiers died from wounds received in what they described as a "hostile attack", but did not give any further information.

NATO reported that two other US troops died on Friday in a bomb attack in southern Afghanistan. Another died from injuries received after a bomb explosion in eastern Afghanistan. The violence comes as US and NATO armed forces have recently began to meet with stronger resistance from Taliban fighters.

Afghan security officers have attacked US soldiers at least two times in 2008. One soldier was killed last October after an Afghan policeman threw a hand grenade at a foot patrol. About a month before that, an Afghan security officer started shooting at a police station, wounding three US soldiers and killing one.

This year has been the deadliest for foreign troops in Afghanistan since troops entered the country almost eight years ago.