A huge manhunt, using helicopters and American ground forces, was under way after the four men were discovered missing from the heavily fortified prison at the Bagram air base, north of Kabul.

American military officials would not comment on the identities of the men but an Afghan official said that they were all from Arab countries and suspected al-Qaeda members. Kabir Ahmed, the district chief of Bagram, identified the men as Abdullah Hashimi, from Syria, Mehmood Ahmed Mohammed, from Kuwait, Mehmood Alfathani, from Saudi Arabia, and Mohammed Hassan, from Libya.

The escape comes less than two weeks after 19 soldiers died in a botched mission in the restive east of the country, 16 of them in a helicopter shot down by Taleban fighters. It was the single biggest combat loss for the Americans since the invasion in 2001.

Dozens of American troops could be seen stopping and searching vehicles yesterday around the sprawling air base and in neighbouring villages while helicopters flew overhead checking for signs of the prisoners.

Lieutenant-Colonel Jerry O’Hara, the military spokesman at Bagram, said: “We consider this very serious business. “These guys are dangerous not only to Afghanistan but to the world in general.” The men were discovered to be missing at 5am local time. Officials said that they could not rule out that they may still be on the sprawling base.

“I can’t give specifics on how they escaped,” Colonel O’Hara said. “The circumstances surrounding the escape are under investigation as we speak.”

Bagram, a former Soviet air base, is guarded by hundreds of troops and has high concrete walls, razor-wire fences and searchlights. There are two main gates, one manned by Afghan guards, the other by American forces.

Access to the prison facility is strictly limited. The spokesman confirmed that it was the first time that anyone had escaped. The prison has housed hundreds of suspected militants since the war to oust the Taleban began in late 2001.

Senior al-Qaeda suspects arrested in neighbouring Pakistan and elsewhere have also been held at the base, where there have been reports of abuse and deaths in custody after torture and beating. About 450 suspects are being held at Bagram. On Saturday 76 Afghan prisoners were released as part of a continuing effort to promote national reconciliation.

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