But defying critics and plunging polls, he declared, "I'm optimistic we'll succeed. If not, I'd pull our troops out."

The president rejected calls for the resignation of Defence Secretary Donald Rumsfeld, chief architect of wars in Iraq and Afghanistan. "Listen, every war plan looks good on paper until you meet the enemy," Bush said, acknowledging mistakes as the United States was forced to switch tactics and change a reconstruction strategy that offered targets for insurgents.

He also rejected assertions by Iraq's former interim prime minister that the country had fallen into civil war amid sectarian violence that has left more than 1,000 Iraqis dead since the bombing last month of a Shia Muslim shrine.

"This is a moment the Iraqis had a chance to fall apart and they didn't," Bush said, crediting religious and political leaders with restraint.

The president spoke for nearly an hour at a White House news conference, part of a new offensive to ease Americans' unhappiness with the war and fellow Republicans' anxiety about fall elections. He faced sceptical questions about Iraq during an appearance Monday in Cleveland, and plans another address soon on Iraq.

Public support for the war and for Bush himself has fallen in recent months, jeopardising the political capital he claimed from his 2004 re-election victory. "I'd say I'm spending that capital on the war," Bush said.

The White House believes that people appreciate Bush's plainspoken approach even if they disagree with his decisions.

"I understand war creates concerns," the president said. "Nobody likes war. It creates a sense of uncertainty in the country."

Bush has adamantly refused to set a deadline for the withdrawal of US forces from Iraq. Asked if there would come a day when there would be no more US forces in Iraq, Bush said, "That, of course, is an objective. And that will be decided by future presidents and future governments of Iraq."

Pressed on whether that meant a complete withdrawal would not happen during his presidency, Bush said, "I can only tell you that I will make decisions on force levels based upon what the commanders on the ground say."

White House officials worried Bush's remarks would be read as saying there would not be significant troop reductions during his presidency. They pointed to comments Sunday by Gen. George W. Casey, commander of US forces in Iraq, who said he expected a substantial troop reduction "certainly over the course of 2006 and into 2007."

The Pentagon announced last December that US force levels would be reduced from the baseline figure of about 138,000 to about 131,000 by the end of March. The total currently is 133,000. In late February the Pentagon told Congress that "it will be possible to consider" additional reductions as the political process moves forward and as Iraqi security forces gain experience. No timetable has been set for deciding on additional cuts.