A top United States official signalled Washington believes Saddam Hussein may be alive as American forces launched an operation to crack down on armed resistance blamed on die-hard supporters of the toppled Iraqi leader.

"I'm assuming he's still alive, and we will get our hands on him, dead or alive," Paul Bremer, who heads the US-led administration in Iraq, told CNN on Sunday.

Asked why it was so hard to find Saddam, Bremer said, "(Iraq is) a big place... He had 30 years to build himself safe houses, palaces, tunnels, we don't know what."

US forces, who declared Saddam's government ousted from power on April 9, attempted during the Iraq war to kill him in bombing raids based on intelligence but have been unable to verify whether they were successful.

American troops backed by warplanes and armoured vehicles launched 'Operation Sidewinder' on Sunday to eliminate armed resistance in areas north of Baghdad where Saddam once enjoyed wide support.

Bremer said US-led forces would suffer further casualties until Saddam loyalists were killed or captured. But US army commander Tommy Franks, who led the swift defeat of Iraq's army, said recent attacks on U.S. troops did not "spoil the victory".

US forces, who have come under fire almost daily in recent weeks in mainly Sunni Muslim central Iraq and sustained soldiers killed, detained more than 60 people and seized weapons and military documents as part of the crackdown.

"No coalition forces casualties were reported in the raids. Sidewinder is...ongoing," US Central Command said in a statement on the mission stretching from the Iranian border to the east to towns north of the capital.

Soldiers also imposed tighter measures around military posts, U.S.-led administration offices and ministry buildings in Baghdad, witnesses said. They also stepped up search operations for weapons and wanted Saddam loyalists.

In the latest of a series of hit-and-run attacks, an Iraqi civilian was killed and two U.S. military police were wounded in Baghdad when an explosion targeted a U.S. convoy.

The United States blames remnants of Saddam's Fidayeen paramilitary force and his Baath Party for the raids. But many Iraqis have warned of widespread discontent if Washington does not quickly restore government to Iraqi hands and rebuild the war-battered nation.

At least 22 Americans have been killed by hostile fire since U.S. President George W. Bush declared major combat over on May 1.

"Will the problems in Iraq and the attacks spoil the victory achieved by the Americans? Of course not," Franks, retiring commander of US Central Command, said after talks with Egyptian President Hosni Mubarak in Cairo.

"It is a certainty that the regime of Saddam Hussein is gone from Iraq... It is also a certainty that some 25 (million), maybe 26 million, Iraqis have a brighter future today than they had three or four months ago," he said.

Three U.S. deaths reported on Saturday took to more than 200 the number of Americans who have died, both in combat and non-combat incidents, since the Iraq war began on March 20.

In Majjar, about 380 km (240 miles) south of Baghdad, there was no sign on Sunday of British forces in the town where gunmen killed six British soldiers last week.

Britain's defence ministry in London said a force of 500 troops returned to Majjar on Saturday where commanders met a delegation of Shi'ite Muslim clerics and local dignitaries.

The troops told people they wanted to help them re-establish their community, not punish them, the ministry said.

Majjah residents said the force, which drove into the town in about 40 military vehicles, stayed for three hours. The British informed town leaders they had no plans to stay, the residents said.

They said the force checked the police station where most of the soldiers died last Tuesday. At least four Iraqis were also killed in the shooting.

"The situation is stable here... We don't need the British," Mohammad al-Shumari, a local dignitary, said.

The Majjah killings were the first British deaths since the toppling of Saddam. Until last Tuesday the British occupation of southern Iraq, populated mainly by the country's majority Shi'ites who were oppressed by Saddam for a quarter of a century, had been largely peaceful.

(Additional reporting Huda Saleh Majeed and Andrew Gray)