BAQOUBA, Iraq — Five Britons captured in Iraq could be used as bargaining tools in attempts by Shiite groups to secure the release of hundreds of prisoners of war being held by coalition troops in Basra, it emerged last night.

A senior official in the Madhi army militia told the Daily Telegraph that the captives — four security guards and a computer expert — had been taken to put pressure on Prime Minister Blair and President Bush to free captives imprisoned in Camp Bucca.

“We are holding the British until they release our brothers from Camp Bucca in Basra,” the cell commander said. “There are hundreds there under British security, some of them for years. When they are released the British will be allowed to go.”

As hundreds of American and Iraqi troops carried out a series of raids in Sadr City — the Madhi army stronghold — it became clear that after they were snatched from a finance ministry building in Baghdad on Tuesday morning, they were immediately driven to a hostage holding center near Sadr City’s Mudafra Square, from where they were expected to be moved around frequently to avoid detection.

The government’s emergency response unit, Cobra, met again yesterday and Margaret Beckett, Britain’s foreign secretary, said officials were working closely with the Iraqi authorities and doing everything they could to secure the captive’s “swift and safe release.”

Mr. Blair, on a visit to Sierra Leone, said: “We know the dangers and challenges there but we shouldn’t let those that are prepared to use kidnapping and terror succeed.”

The Foreign Office said there was “no firm indication yet” as to who was behind the abductions.

The Mahdi army official said the order to seize the hostages was handed down by Hassan Salim, the army’s leading figure.

He said the group is seeking to emulate what it sees as the successful outcome of the recent seizure of the 15 British sailors captured the Shatt al Arab waterway by its allies in the Iranian government, adding that the militia’s demands had already been passed on to Iraq’s Prime Minister al-Maliki, a Shiite Muslim politician close to the Madhi army’s political wing.

Last night, there was no official confirmation any demands had been made.

However, he was directly contradicted by Sheikh Abdel al-Sattar al-Bahad, a senior aide to the radical Shiite cleric Moqtada Sadr, to whom the Madhi army is intensely loyal, who denied any involvement by the Madhi army.