AMARA, Iraq (Reuters) – Iraq’s government beefed up army and police units in the southern city of Amara on Sunday for a new crackdown on Shi’ite militias, witnesses said.

Convoys including armored vehicles and tanks were moving through the northern side of the city, said a Reuters reporter.

The operation, which officials say will start on Thursday, is the latest stage in Prime Minister Nuri al-Maliki’s drive to stamp his government’s authority on areas previously controlled by Shi’ite militias or Sunni Arab insurgents.

Army Major-General Tareq Abdel Wahab, leader of the security operation, told Reuters that government forces had a list of hundreds of “outlaws, criminal gangs and those who violate security” it would hunt down.

He said a small number of U.S. forces were available if air cover and logistical support are needed.

Amara is a stronghold of Shi’ite cleric Moqtada al-Sadr, who agreed to a ceasefire after U.S.-backed Iraqi forces launched a major offensive on his Mehdi Army militia in Basra in March.

Maliki, perceived by some as lacking the resolve and charisma needed to stabilise Iraq, has gained respect at home and abroad with security crackdowns that have helped reduce violence to the lowest level in over four years.

Iraqi-led operations underscore his Shi’ite-led government’s desire to take more control of security from the 150,000 U.S. troops in Iraq.

U.S. President George W. Bush urged British Prime Minister Gordon Brown on Sunday not to set a timetable for the withdrawal of troops from Iraq.

He said in an interview with Britain’s Observer newspaper that the United States and Britain, Washington’s main ally on Iraq, both obviously wanted to bring their troops home but this could only be “based upon success.”

“Our answer is: there should be no definitive timetable,” said Bush, adding he was “appreciative” that Brown was in frequent touch about “what he and his military are thinking.”

The newspaper described Bush as issuing a warning to Brown, but the White House dismissed that tone, saying there was no disagreement between the United States and Britain on Iraq.

Only about 4,200 British troops are still in Iraq, most of them stationed at a base in the south. They are the largest foreign military contingent in Iraq after U.S. forces.

Britain has indicated that it could pull all forces out by the end of the year, but with the situation still unstable that appears unfeasible.

AMARA RESIDENTS NERVOUS

In a statement read on state television on Saturday night, Maliki said he was giving outlaws and criminals a last chance to surrender and hand over heavy and medium weapons in four days in Maysan province. Amara is the regional capital.

Checkpoints have been set up on streets in Amara, home to about 250,000 people, 300 km (185 miles) southeast of Baghdad.

Helicopters dropped leaflets urging residents in the city to stay home. U.S. forces are likely to play a backing role, such as providing helicopter air support.

Hazim al-Araji, who heads a Sadr delegation negotiating with the government over the security operation, said Mehdi Army fighters would not be turned over without a court order.

Abdel Wahab said the military held talks with Sadr’s representatives on security cooperation.

While Iraqi military officials reviewed last-minute plans, the impoverished residents of Amara feared for the worst.

“We are very scared of this operation because the battle will destroy the town. Most people won’t leave their houses,” said Hassan Hameed, 35, a day laborer.

“We will not accept the targeting of the Medhi Army because they provide people with services.”

U.S. forces, which invaded Iraq in 2003, are gradually handing over more security responsibilities to Iraqi forces.