US Defence Secretary Donald Rumsfeld says some "seasoned" US troops in Iraq might be kept there longer than planned to deal with the growing violence.
Accounts vary about casualties in Falluja
The troops have not lost control of the security situation, despite a recent upsurge in fighting, Mr Rumsfeld said.
The clashes were the work of a few "thugs, gangs and terrorists", he said.
In the latest reported violence, US troops were involved in clashes with both Sunni and Shia insurgents in the north and west of the capital Baghdad.
Operations were also continuing in the Sunni city of Falluja west of the capital - a day after the US military bombed a compound housing a mosque.
Iraqi witnesses say about 40 Iraqis were killed in the strike, but US Central Command said only one "anti-coalition force member" had died and there were no civilian casualties.
It is not known if there were any deaths among the five casualties reported by the US military.
Several days of clashes have claimed the lives of well over 100 Iraqis and at least 30 coalition soldiers in what is being seen as the worst escalation of fighting since Baghdad fell to US-led forces a year ago.
The involvement of Shia fighters loyal to radical cleric Moqtada Sadr has opened a second front in the Iraqi opposition to the occupation.
Iraq's most senior Shia cleric, Ayatollah Ali al-Sistani, has condemned the way US forces are dealing with Shia resistance.
Mr Sistani appealed for a wise and peaceful approach from all sides.
In other developments:
- A convoy of Iraqis is reported to be heading for Falluja to take humanitarian aid to the besieged city.
- For the first time on Wednesday, Japanese troops in the southern town of Samawa were apparently targeted by a number of explosions. There are no reports of injuries.
- Italian troops in the southern city of Nasiriya withdraw to allow Iraqi police to take control.
Speaking in Washington, Mr Rumsfeld said US troops faced a "serious problem", but "the problem's being worked".
"The number of people that are involved in those battles are relatively small," Mr Rumsfeld said.
But there was "nothing like an army or... large elements of hundreds of people trying to overthrow or to change the situation," the defence secretary said.
The city of Najaf was the only part of Iraq not under the control of US and coalition forces, he said.
"We will likely be managing the pace of the redeployments to allow those seasoned troops with experience and relationships with the local populations to see the current situation through," he added.
BBC Pentagon correspondent Nick Childs said the defence secretary's language and the hurried news conference may be a sign of how worried the Bush administration is about what is unfolding in Iraq.
The US military has vowed to "destroy" the Shia Muslim Mehdi Army, set up last June by Moqtada Sadr.
Mr Sadr has been named in an arrest warrant, accused of inciting the murder of a rival cleric and two colleagues last April.
He has issued a statement from Najaf calling for power in Iraq to be handed over to "honest men" and not to collaborators of the US-led occupation.