The senior British general in Iraq has been negotiating a deal with the Mehdi Army, the country's most powerful religious militia, to prevent armed clashes with allied forces from Baghdad to Basra.

The UK deputy commander of all coalition troops in Iraq, Lieutenant-General Graeme Lamb, has been trying to defuse tensions over a potential showdown triggered by US-led incursions into Shi'ite strongholds in the capital.

Negotiations began after the White House announced a reinforcement of more than 20,000 US troops aimed at stamping out lethal sectarian violence between Sunnis and Shias, which threatens to drag Iraq into civil war.

General Lamb has met representatives of renegade cleric Moqtada al Sadr at least twice in the past two weeks in efforts to reach an agreement taking militia weapons off the streets of Sadr City, a Shi'ite slum area of the capital with two million inhabitants.

The district is protected by up to 7000 Mehdi fighters who man their own roadblocks to defend against attacks by rival Sunni gunmen and impose the Shi'ite brand of justice on neighbourhoods under their control.

Rahim al Darraji, Sadr City's mayor, said yesterday that agreement had been reached between "all militia groups" that firepower would not be flaunted on the streets as long as coalition units guaranteed the security of the sprawling slums. He added that there would be no problem if Iraqi government forces confiscated unlicensed weapons.

Urging the recruitment of locals from the impoverished suburb for the police and army, al Darraji said a key precondition was that stability would be maintained as long as coalition troops did not attempt to establish an "armed presence" in Sadr City.

A key part of the White House blueprint for imposing peace by flooding Baghdad with fresh troops was the siting of battalion-sized security bases in the heart of each clearly-defined sectarian neighbourhood.

Visible Mehdi Army activity has fallen in the past two weeks out of concern that the streets would become a battleground where Shi'ite civilians would be the main victims.

A spokesman for Gen Lamb, who previously commanded a British infantry division, confirmed he had met "various community leaders in the course of his duties" but refused to disclose the nature of the discussions. "We respect the confidence of those meetings," the spokesman said.

Sadr City has been targeted heavily by Sunni bombers in recent months, including one attack on November 23 which killed more than 200.

About 400 Mehdi militia are currently in custody suspected of carrying out or organising retaliatory attacks on Sunni areas. US and Iraqi forces have captured 16 high-level Mehdi men and 33 equivalent-rank Sunni insurgents in the past 45 days. Another 600 Mehdi militiamen are in jail awaiting prosecution by the Iraqi government.

Military intelligence estimates that there are as many as 60,000 armed members of the group throughout Iraq.

The Mehdi militia was defeated in 2004 when it staged two mass uprisings.

The violence spread to the British-controlled south and led to pitched battles in al Amara and Basra.