"We are holding the British until they release our brothers from Camp Bucca in Basra," a cell commander with the Mahdi Army told The Telegraph. "There are hundreds there under British security, some of them for years. When they are released the British will be allowed to go."

The Britons, four security guards and a computer expert, were seized Tuesday from a Finance Ministry building in Baghdad.

The Mahdi Army commander told the Telegraph that the hostage-taking was ordered by militia leader Hassan Salim. But Sheik Abdel al-Sattar al-Bahad, a senior aide to rebel Shiite cleric Moktada al-Sadr, denied that the Mahdi Army was involved, the newspaper said.

U.S. and Iraqi troops searched Shiite City Wednesday for the Britons, and British Foreign Secretary Margaret Beckett said officials were working closely with the Iraqis to secure the captives' "swift and safe release."