President George W Bush says the US military is prepared to use "decisive force" in Iraq, where the Americans are facing insurgents on two fronts.
Bush said US troops would remain in Iraq as long as needed
He spoke as 2,500 US soldiers prepared for action against armed supporters of radical Shia cleric Moqtada Sadr in the southern holy city of Najaf.
The speech came as more US casualties were reported in the besieged city of Falluja, west of Baghdad.
Meanwhile Russia says it is evacuating its nationals from Iraq.
In a live televised speech followed by a question-and-answer session, President Bush said the renewed violence across the country did not reflect the feelings of most Iraqis.
"This is not a popular uprising," Mr Bush said.
A large US force backed by tanks and artillery has gathered on the outskirts of Najaf.
1 - Sporadic clashes between Sunni fighters and US troops in Falluja
2 - Russia set to airlift nationals out of Baghdad
3 - US troops poised on outskirts of Shia city of Najaf
On Wednesday Iran - a predominantly Shia country - said it had sent a top foreign ministry official to Iraq to help mediate in the crisis.
Mr Sadr has barricaded himself near a shrine in Najaf, and vowed to continue the "popular revolution" against occupying troops.
"I fear only God," he told a Lebanese TV station on Tuesday. "I am ready to sacrifice my blood for this country."
US officials have vowed to kill or capture him.
"The target is not Najaf. The target is Moqtada al-Sadr and his militia," said Gen Mark Kimmitt, deputy head of US military operations.
Iraqi politicians and clerics have been engaged in a mediation attempt to avert a US assault on Najaf.
An envoy appointed by Mr Sadr said the cleric had asked him to convey peace proposals to the coalition - but he gave no details.
Colonel Robert Strzelecki - a Polish officer who is spokesman for the multinational force responsible for Najaf - told the BBC that Iraqi security forces were controlling the city and coalition troops were helping patrol its streets.
He said the troops under his command would not take part in any offensive operations.
The BBC's Barbara Plett in Baghdad says most Iraqis fear the use of overwhelming US force, and would prefer a political rather than a military solution.
In Falluja a fragile ceasefire agreed on Sunday has been extended for another 48 hours to allow hospitals to care for the wounded, mediators say.
The truce has been strained by sporadic fighting in recent days.
The US Army says four marines were killed in western Iraq on Monday and Tuesday.
With the latest killings, April has become the deadliest month for the US military since the Iraq war began in March last year.
At least 87 US soldiers have died in the past two weeks while aid agencies counted at least 470 Iraqi dead in the city of Falluja alone last week.
Meanwhile Moscow has announced it is evacuating more than 800 citizens from Russia and other former Soviet countries, following a spate of kidnappings.
The Russian government says it will send planes to Baghdad on Thursday and Friday to pick them up.
The Americans have their sights set on Sadr
The announcement followed the capture on Monday of three Russians and five Ukrainians, who were later released.
According to the coalition, 40 foreign hostages are currently being held.
Japan - which is trying to secure the release of three of its nationals being held hostage - has urged its remaining citizens to leave Iraq.
The BBC's Caroline Hawley in Baghdad says the recent abductions have given many the impression that no foreigner is safe in Iraq.
Four mutilated bodies have been found west of Baghdad, the US State Department said.
The bodies have not been identified but there is speculation that they could be US contractors missing since an ambush on their fuel convoy last Friday.