Two US nationals working for the CIA in Afghanistan were killed in an ambush at the weekend, the American intelligence agency has said in a rare statement.
William Carlson and Christopher Glenn Mueller were both working under contract for the agency, "tracking terrorists" in the east of the country, near the border with Pakistan.
There are frequent clashes between US-led forces and rebel fighters
The statement said that the agency had decided releasing the names of the dead men would not compromise operations.
They died near a village called Shkin in Paktika Province - an area which is believed to be a hideout for members of the al-Qaeda network.
The announcement brings to four the number of operatives whom the agency has acknowledged have been killed since the attacks of 11 September.
'Bin Laden' territory
It is unclear whether the men were killed during a period of fighting at the weekend in the same region between US-led troops and rebel fighters, which began on Saturday morning.
Remnants of the Taleban, which ruled Afghanistan until the US launched military action in 2001, and members of al-Qaeda reportedly continue to mount attacks in the inhospitable border region.
There has long been speculation that Osama Bin Laden is hiding in this area.
The CIA did not provide details of the circumstances in which Mr Carlson and Mr Mueller died.
The two men were veterans of military special operations forces,
the CIA said, and working for the agency's Directorate of
Operations, which conducts clandestine
intelligence-gathering and covert operations.