General Richard Myers fired the opening salvo of the campaign, on BBC television in London early Sunday.

"The toughest fighting is ahead of us," said the chairman of the Joint Chiefs of Staff, amidst criticism that US President George W. Bush and his aides had given overly optimistic estimates of Iraqi resistance and the length of the war.

"I'm not going to make a prediction in terms of how long this conflict will last," said Myers.

Defense Secretary Donald Rumsfeld was also scheduled for interviews on two Sunday news shows.

Bush said Saturday that US and British forces were advancing steadily on Baghdad in the face of fierce fighting by "the most desperate" Iraqi units.

But US forces were largely dug in over the weekend, waiting for their supply lines to catch up.

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.

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

The Washington Post on Sunday defended Bush against rising criticism of the war's slow pace by pointing to battlefield successes. Troops secured oil fields and Iraq's only seaport where humanitarian aid was unloaded -- while keeping Israel, Turkey and the Kurds out of the war.

"From outside the theater it's impossible to judge whether a field officer's predictions that the war could last past summer and require tens of thousands of additional troops will prove accurate.

"But if such a commitment is needed to ensure success, the United States should make it," the Washington daily said.

"Polls show that most Americans are prepared to accept a longer war -- whatever their expectations beforehand -- and no battlefield setback could rival the damage to US security were Washington to falter in combating Saddam Hussein's most fanatical supporters.

"It must stay the course."

A Washington Post-ABC News survey published Saturday 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 Saturday 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."

Some 25,000 anti-war marchers clogged the streets of Boston, Massachusetts Saturday in what may have been the largest demonstration there in decades, and more than 8,000 protested in Harrisburg, Pennsylvania.

In a change of pace, a small but feisty pro-troops rally was staged in front of city hall in usually-peace-minded San Francisco, California.

WAR.WIRE