KABUL, Afghanistan -- Two Americans working for the CIA have been killed in an ambush while tracking terrorists in Afghanistan, the agency said Tuesday.

The ambush Saturday happened on the same day and in the same region as a six-hour firefight in which U.S.-led coalition aircraft and Afghan militia killed 18 rebel fighters, the U.S. military reported from its headquarters in Afghanistan Tuesday.

Six Afghan militia soldiers were wounded in the fighting, but there were no coalition casualties, the military said. It was unclear whether the two incidents were linked, but the military did not explain why its account of the fighting was delayed by three days.

In Washington, the CIA identified the two men as William Carlson, 43, of Southern Pines, N.C., and Christopher Glenn Mueller, 32, of San Diego. Both were veterans of military special operations forces, the CIA said.

They were "tracking terrorists operating in the region" of Shkin, a village in eastern Afghanistan, when they were killed Saturday, the CIA said in a statement.

The pair was working for the CIA's Directorate of Operations, which conducts clandestine intelligence-gathering and covert operations.

The agency did not provide details of the ambush or the two operatives' mission.

The area they were operating in is part of the remote mountainous region along the border of Afghanistan and Pakistan, where Osama bin Laden is thought to be hiding. It also is a stronghold for al-Qaida, Taliban and other anti-U.S. fighters.

A statement in Afghanistan said U.S.-backed Afghan militia encountered as many as 25 anti-coalition forces Saturday while patrolling 27 miles south of a base in Shkin in the volatile Paktika province.

A rapid reaction force from Shkin base, 135 miles south of Kabul, was called in to reinforce the Afghan soldiers, the statement said.

A-10 Thunderbolt airplanes and Apache helicopters conducted air strikes while ground forces exchanged small-arms fire with the attackers, it said. One vehicle was destroyed, and the surviving rebels retreated. It said "approximately 18 enemy personnel" were killed.

The clash was reported Monday by Afghan officials, but they gave conflicting accounts. Tuesday's statement was the first by the coalition on the incident.

Mohammed Ali Jalali, governor of Paktika province, said Tuesday a separate battle Saturday in the province's Gomal district, about two miles from the Pakistan border, left 10 rebels dead -- including four Arabs.

The coalition statement didn't specify if the attackers were former Taliban or al-Qaida terrorists. Remnants of those forces -- ousted from power in late 2001 by the coalition -- have mounted increasing attacks on coalition forces and their Afghan allies.

Carlson and Mueller are the third and fourth CIA operatives that the agency has acknowledged have been killed in Afghanistan in the line of duty since the Sept. 11 attacks.

"William Carlson and Christopher Mueller were defined by dedication and courage," CIA Director George J. Tenet said in a statement. "Their sacrifice for the peoples of the United States and Afghanistan must never be forgotten."

The CIA statement said the agency consulted with the dead officers' families and decided their names could be released without compromising ongoing operations.

The first CIA casualty, paramilitary officer Johnny Micheal Spann, was killed during an uprising of Taliban and al-Qaida prisoners in northern Afghanistan on Nov. 25, 2001.

The second, Helge Boes, died in a training accident in eastern Afghanistan, on Feb. 5, 2003.

The remote regions on Afghanistan's frontier have poor communication and transport links, a possible reason for the delay and confusion about the battles.

Last week, U.N. Undersecretary-General for Peacekeeping Jean-Marie Guehenno told the U.N. Security Council that deteriorating security in Afghanistan was a significant obstacle to reconstruction. He claimed that the Taliban have established "de-facto control" in certain border areas, including in Paktika province, site of Saturday's fighting.

The Afghan government strongly rejected the U.N. official's claims the Taliban have taken control of border regions, and said threats to stability in the country shouldn't be exaggerated.