US forces have launched a major offensive against resistance fighters in central Iraq.
US troops have been setting up checkpoints
Operation Sidewinder began with more than 20 simultaneous raids involving aircraft, armoured vehicles and infantry in an area north of the capital, Baghdad, along the River Tigris.
The US military says at least 60 people have been detained and illegal weapons and military documents have been seized.
It is the third such operation in recent weeks and follows a spate of deadly attacks on US forces in Iraq, which the Americans say are being directed by supporters of former President Saddam Hussein's Baath Party.
We go in with such overwhelming combat power that they won't even think about shooting at us
Lieutenant Colonel Mark Young
News of the offensive came after the chief US administrator in Iraq said the failure to capture or kill the deposed Iraqi president was hampering coalition efforts to control the country.
US soldiers in the town of Fallujah have been confiscating motorbikes, apparently to prevent guerrilla-style attacks, a correspondent for AFP news agency reported.
US troops suspect many of the attacks against them have been carried out using motorbikes, an Iraqi police officer said.
In a statement, US Central Command said Operation Sidewinder was designed to "root out elements attempting to undermine coalition efforts to restore basic infrastructure and stability in the region".
"We go in with such overwhelming combat power that they won't even think about shooting at us," Lieutenant Colonel Mark Young said before the start of the operation, which was expected to last several days, according to military officials.
In another statement, Central Command said 15 people were arrested in Mosul, in northern Iraq, on Saturday. Documents, weapons and Republican Guard uniforms were confiscated in the raids.
In a BBC interview, the chief US administrator in Iraq, Paul Bremer, said the chances of catching Saddam Hussein were "very high".
Since end of hostilities:
Killed in combat:
US - 24, UK - 6
US - 39, UK - 4
But he said the failure to capture or kill him thus far meant attacks on coalition forces - which have killed at least 30 troops since the end of the war - were likely to continue for some time.
"The fact that we have not been able to show his fate allows these remnants of the Baathist regime to go around... and say Saddam will come back, and we will come back, so don't co-operate with the coalition," he said.
"Unfortunately it is the case that we will continue to take casualties... but there's no strategic threat to the coalition,
"We are going to fight them and impose our
will on them and we will capture or, if necessary, kill them until we have imposed law and order on this country."
Mr Bremer, who was appearing on the BBC's Sunday morning news programme Breakfast With Frost, rejected criticism that the US lacked a strategy for reconstructing Iraq.
He said the first priority for establishing a new government was to rewrite the Iraqi constitution to allow democratic elections to take place. With that in mind, a constitutional council would meet in July.
And he said progress was being made on rebuilding infrastructure.
He listed achievements in restoring electricity to pre-war levels, getting all the country's hospitals and 95% of its health clinics operational, improving the water supply in the second city, Basra, starting a vaccination programme and distributing food.
"There's a lot of good news that tends to get lost in the noise here," he said.