KABUL, Afghanistan — Four suspected insurgents escaped Monday from the main U.S. base in Afghanistan, the first time anyone has broken out of the heavily guarded detention facility, sparking a massive ground and air search, officials said.

"They are considered dangerous and are suspected terrorists,'' a U.S. military spokeswoman, Lieut. Cindy Moore, told The Associated Press.

The four are Arabs from Syria, Saudi Arabia, Kuwait and Libya, said Kaber Ahmad, the government chief in Bagram, which is adjacent to the vast U.S. base by the same name, and whose security forces are helping in the search.

"Coalition forces, police and Afghan troops have surrounded several villages near the base,'' Ahmad said. Photos of the four, who have short hair and long beards and were wearing yellow prison clothes, were distributed, he said.

Moore declined to identify the four or elaborate why they were being held. Another military spokesman, Lt.-Col. Jerry O'Hara, described them only as "enemy combatants.''

He said it was the first time anyone has broken out of Bagram's detention facility. The base is home to thousands of U.S. and coalition soldiers.

U.S. helicopters, American troops on the ground and Afghan forces were scouring the area around Bagram, an hour's drive north of Kabul, for the four, who vanished around dawn, she said.

About 500 people are being held by U.S. forces in Afghanistan, most of them at Bagram, O'Hara said.

Some 76 detainees who were no longer considered to be a threat were released from U.S. detention facilities Saturday, a week after an initial group of 57 were set free. Another 66 are to be freed shortly, a U.S. military statement said Monday.

Though no one has escaped from Bagram, detainees have broken out of other prisons. In October 2003, 41 suspected Taliban rebels escaped from an Afghan government-run jail in the southern city of Kandahar by digging a tunnel.

The escape comes after allegations that U.S. military personnel at Bagram and at other detention facilities have abused prisoners.

U.S. military spokesmen have said the military does not tolerate maltreatment of prisoners.