US-led troops are continuing a huge manhunt for four "dangerous enemy combatants" who have escaped from the main US base in Afghanistan.
The prisoners are the first to escape from Bagram
Helicopters were helping ground forces and local officials' efforts to search for the men who escaped from the Bagram air base, north of the capital, Kabul.
The US says it is the first time any prisoner has escaped from Bagram.
Hundreds of detainees, including Afghan nationals and foreign al-Qaeda suspects, are held at the air base.
US soldiers have set up checkpoints around the base after the four suspected militants escaped from a detention centre at Bagram on Monday.
"There's a pretty large search operation ongoing with coalition forces and the Afghan national army," US military spokesman Lt-Col Jerry O'Hara told the AFP news agency.
"It's a very serious issue, because they escaped from our facility. We take it very seriously, and want to take them back to the facility where they belong."
Col O' Hara said the men were wearing "orange clothes, which make them more recognisable, but apparently they might have changed them".
The US military said the men who escaped were militants from Syria, Kuwait, Saudi Arabia and Libya.
Some reports on Tuesday said one of the four had been rearrested in a mosque near the airbase.
However, a high ranking Afghan security official told the BBC on condition of anonymity that the man held was an Afghan national who had been detained by mistake.
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.
The detention facility is said to house senior al-Qaeda suspects from countries such as Pakistan.
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.
Coalition forces check a wedding car during the manhunt
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.