Four "dangerous enemy combatants" have escaped from the main US base in Afghanistan, the US military has said.
US soldiers spread out around Bagram to try to catch the fugitives
A huge manhunt was launched around the Bagram air base north of the capital Kabul, after the men, said to be Arabs, escaped at about 0500 (0030 GMT).
The US says it is the first time any prisoner has escaped from Bagram.
Hundreds of detainees, most of them Afghan nationals but a number of senior foreign al-Qaeda suspects, are held at the detention centre.
The US military said the men who escaped were militants from Syria, Kuwait, Saudi Arabia and Libya.
Helicopters were assisting ground forces and local Afghan officials in search efforts.
"We have on operation to recover four individuals who escaped from our detention facility," said US military spokesman Lt-Col Jerry O'Hara. He would not say how the men escaped.
US troops have set up checkpoints on roads around the base and are searching vehicles.
The two-storey building that houses prisoners is deep inside the Bagram facility.
The BBC's Andrew North in Kabul says the prisoners would have had to negotiate various checkpoints to reach the perimeter.
There are reports from Afghan sources that all four escaped detainees are Arabs.
Col O'Hara refused to comment but a government official in neighbouring Bagram town said coalition forces had handed out photographs of the four men.
"They all have short hair, long beards and are wearing yellow prison clothes," said Kaber Ahmad.
The detention facility is said to house senior al-Qaeda suspects from countries such as Pakistan.
The prison at the Bagram air base houses senior al-Qaeda suspects
Col O'Hara said there were no US casualties in the escape.
The Bagram facility has proved controversial.
US soldiers there have been accused of a number of abuses of prisoners - including torture and the deaths of two inmates - sparking criticism from human rights groups and a thorough investigation by the US military.
The details of the investigation were leaked to the New York Times in May.
Afghan President Hamid Karzai said he was shocked by the report and called for the guilty to be punished.
Seven US servicemen have been charged in relation to the two deaths at Bagram in 2002.
Monday's escape came as the US pledged another 700 extra troops to bolster security ahead of September's parliamentary elections.
The airborne battalion would arrive "very soon", said US military spokesman Col James Yonts.