US and Iraqi security forces have conducted a search operation in the slums of Sadr City - a bastion of Shi'ite insurgents. The U.S. and Iraqi forces searched a number of houses on Tuesday (March 6), looking for militants from the Mehdi army, supporters of the radical anti-American Shi'ite cleric Moqtada al-Sadr. On Sunday (March 4), more than 1,000 U.S. and Iraqi troops launched an incursion into a Shi'ite militia bastion in Baghdad, meeting no resistance as they searched homes for illegal weapons and carried out patrols. The operations in Sadr City could test Iraqi and U.S. determination to enforce a security crackdown regarded as a last attempt to stop Iraq sliding into all-out sectarian civil war. A U.S. military statement said 600 American and 550 Iraqi security forces backed by American Stryker armoured vehicles took part in the operation. It said no one was detained nor any weapons caches found. There was no violence. The young firebrand Sadr, a key supporter of Shi'ite Prime Minister Nuri al-Maliki, has criticised the crackdown and said it will fail as long as U.S. forces are involved. Raids had been expected in Sadr City after U.S. and Iraqi commanders met community leaders last week to give them advance warning of the incursions and to seek their support. Many members of the Mehdi Army are either lying low or have left Baghdad, unlike in 2004 when the militia twice rose up against American forces. Washington calls the Mehdi Army the greatest threat to Iraq's security. Some Shi'ite officials outside Sadr's movement say the militia wants to avoid a battle to protect the cleric's political gains. Sadr's movement holds a quarter of the parliamentary seats in Maliki's ruling Shi'ite Alliance. Lieutenant-General Raymond Odierno, the number two U.S. commander in Iraq, said around 700 Shi'ite militants had been arrested in the past two months. Many were Mehdi Army members, he told CNN.