BAGRAM, Afghanistan (Reuters) – Four dangerous Arab al Qaeda militants escaped from detention at the main U.S. base in Afghanistan on Monday, officials said, as U.S. forces struggle with a worsening Islamic insurgency.

A major search, involving ground forces and helicopters, was launched after militants from Syria, Kuwait, Saudi Arabia and Libya were reported missing from the detention centre at Bagram Air Base to the north of Kabul early in the morning.

U.S. military spokesman Lieutenant-Colonel Jerry O”Hara described the men as &#34dangerous enemy combatants&#34, but declined to identify them.

Kabir Ahmad, chief of Bagram district, said they were Syrian Abdullah Hashimi, Kuwaiti Mahmoud Ahmad Mohammad, Saudi Mahmoud Alfatahni and Libyan Mohammad Hassan.

The escape is the first known from the heavily guarded detention centre, 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.

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

&#34We consider this very serious business,&#34 O”Hara said. &#34These guys are dangerous not only to Afghanistan but to the world in general.&#34

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.

&#34I can”t give specifics on how they escaped,&#34 he said. &#34The circumstances surrounding the escape are under investigation as we speak.&#34

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.

Ahmad said he had heard the men may have escaped the base by car. &#34We are working to find out how they escaped,&#34 he said. &#34If they are in this area, then there”s a strong possibility we will arrest them.&#34

Photographs were circulated to Afghan security forces showing bearded men with shaved heads wearing orange prison uniforms.

The Bagram detention centre 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 September 11 attacks on the United States.

It has also housed senior al Qaeda suspects arrested in neighbouring 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 September 18 parliamentary elections, the next big step in Afghanistan”s difficult path to stability.