Backed by polls showing US support for a war even with heavier casualties, Bush also accused Iraqi President Saddam Hussein's forces of atrocities and promised to hunt down and punish war criminals.

"We are now fighting the most desperate units of the dictator's army," Bush said in his weekly radio address from the Camp David presidential retreat in Maryland.

"The fighting is fierce and we do not know its duration, yet we know the outcome of this battle: The Iraqi regime will be disarmed and removed from power. Iraq will be free," he said.

Bush, speaking from Maryland's Catoctin Mountains, 100 kilometersmiles) north of the White House, said US and British troops were nearing Baghdad and had secured more than 600 oil wells.

"Thanks to our fighting forces, the regime that once terrorized all of Iraq now controls a small portion of that country. American and coalition troops have continued a steady advance, and are now less than 50 mileskilometers) from Baghdad," he said.

Bush and his top aides have vigorously defended their war strategy after a senior US field commander, Lieutenant General William Wallace, said the combat could drag on.

Critics said the US battle plan underestimated Iraqi resistance and guerrilla-type tactics, and that the US and British forces were running into difficulties supplying troops.

But the Pentagon insisted only fear of President Saddam Hussein's regime was preventing an uprising of Iraqi people.

"I think it is perfectly understandable why so many of these people are afraid to rise up against the regime at this time," said Defense Department spokeswoman Victoria Clarke.

"When they are as certain as we are about the end of this regime, I am confident they will do so," she told a news conference

Major General Stanley McChrystal, vice director of operations for the Joint Chiefs of Staff, said forces were getting sufficient food, water and munitions.

"The big answer is: No, there is not a resupply problem," he said.

Despite the questions, polls showed overwhelming popular US support for the war.

A Washington Post-ABC News survey found 74 percent of those asked backed the war, up from 72 percent a week earlier.

The proportion of respondents expecting a "significant number of additional US casualties" shot up to 82 percent from 37 percent a week beforehand, the poll said.

Some 63 percent of those questioned in a separate Newsweek poll taken Thursday and Friday said the United States was right to start military action when it did. And 68 percent approved of Bush's job performance -- up 15 percentage points from two weeks ago.

A suicide bomber at a checkpoint in central Iraq underscored the difficulties US troops were encountering.

Four US soldiers were killed when a taxi driver blew up his vehicle at a checkpoint north of the Shiite Muslim holy city of Najaf, some 150 kilometers (90 miles) from the capital, a US military spokesman said in Qatar.

Iraqi state television said the bomber was an Iraqi army officer seeking to teach the Americans a "lesson."

Bush charged that Saddam's regime brutalized and executed prisoners of war, murdered Iraqis who refused to fight and hanged an Iraqi woman who waved at US and British troops.

Iraqi troops pretended to surrender, only to open fire on their captors, he said.

"Given the nature of this regime, we expect such war crimes, but we will not excuse them. War criminals will be hunted relentlessly and judged severely," Bush said.

"Every atrocity has confirmed the justice and urgency of our cause. Against this enemy, we will accept no outcome but complete and final victory," he said.

Bush, who rarely spends weekends at the White House, was accompanied at Camp David by his wife, Laura; national security adviser Condoleezza Rice; and White House chief of staff Andrew Card.

Small-scale protests and pro-troop rallies were expected in the United States over the weekend.

Anti-war movement organizers focused their energies on preparing for a worldwide day of protest on April 12.

WAR.WIRE