A major search, involving ground forces and helicopters, was launched after the men were reported missing from the detention center at Bagram Air Base to the north of Kabul, the military said in a statement.

Arabic television channel al Jazeera quoted unidentified sources as saying the four men were Arabs, but U.S. military spokesman Lieutenant-Colonel Jerry O'Hara declined to confirm this, describing them only as "dangerous enemy combatants."

The escape is the first known from the heavily guarded detention center, which is within the sprawling Bagram base, and as such a major embarrassment for the U.S. military. The men were reported missing at about 5 a.m. (0030 GMT).

An al Jazeera correspondent, quoting Afghan government sources, later told a news bulletin the escapees were al Qaeda members.

Dozens of U.S. troops could be seen stopping and searching vehicles every few hundreds meters (yards) around the base and nearby villages, while U.S. helicopters flew overhead.

"We consider this very serious business," O'Hara said. "These guys are dangerous not only to Afghanistan but to the world in general."

He said there had been no U.S. casualties in the escape and he had no reports of violence or any U.S. personnel missing.

"I can't give specifics on how they escaped," he said. "The circumstances surrounding the escape are under investigation as we speak."

O'Hara said the men, who would normally wear orange prison uniforms, could still be on the massive base, about 50 km (30 miles) north of the Afghan capital Kabul, and it was being thoroughly searched by military police.

The detention center has housed hundreds of militant suspects since U.S.-led forces overthrew the Taliban in late 2001 for refusing to give up al Qaeda leader Osama bin Laden after the Sept. 11 attacks on the United States.

It has also housed senior al Qaeda suspects arrested in neighboring Pakistan and elsewhere.

A U.S. military spokeswoman said at the weekend about 450 militant suspects were being held at Bagram.

Monday's escape follows a painful episode for U.S. military in the eastern province of Kunar in which it suffered its worst losses in a single combat operation in Afghanistan.

Sixteen U.S. special forces troops died when militants shot down their helicopter on June 28 during a mission to rescue a four-man Navy SEAL reconnaissance team trapped in a firefight.

Three members of the team were killed and one escaped.

The losses have made 2005 the bloodiest year for U.S. forces in the country and have come amid stepped-up militant violence ahead of Sept. 18 parliamentary elections, the next big step in Afghanistan's difficult path to stability.

© 2005 Reuters

###