US President George W. Bush on Tuesday reiterated his refusal to set an "artificial" time table for withdrawing US troops from Iraq and said the decision must depend on conditions on the ground.

The president, who is facing plummeting ratings amid increasing public dissatisfaction with the war in Iraq, remained vague about what the right conditions would be as the Iraqi government presses for a pullout timeline.

"The Iraqis have invited us to be there. But they share a goal with us, which is to get our combat troops out, as conditions permit. Matter of fact, that's what we're doing. Return on success has been the strategy of this administration, and our troops are coming home, but based upon success."

Asked by a reporter what his advice would be to Democratic White House hopeful Barack Obama, who is expected to embark on a trip to Iraq later this month, Bush said Obama should heed the advice of military commanders.

"I would ask him to listen carefully to (Ambassador) Ryan Crocker and General (David) Petraeus. There's a temptation to let the politics at home get in the way with the considered judgment of the commanders.

"That's why I strongly rejected an artificial timetable of withdrawal," Bush said in his press conference, which began about one hour before Obama delivered a major foreign policy speech of his own.

"It's kind of like an arbitrary thing, you know -- 'We will decide in the halls of Congress how to conduct our affairs in Iraq based upon polls and politics, and we're going to impose this on people' -- as opposed to listening to our commanders and our diplomats, and listening to the Iraqis, for that matter," Bush said.

Bush was asked about his reaction to Obama's remarks, of which excerpts were released to the media in advance, saying that "the central front in the war on terror is not Iraq and it never was."

Bush responded: "Well, as you know, I'm loathe to respond to a particular presidential candidate, and so I will try not to.

"One front right now is going better than the other, and that's Iraq, where we're succeeding, and our troops are coming home based upon success. And Afghanistan is a tough fight," Bush said.

"And so I would ask whoever goes there, whatever elected official goes there, to listen carefully to what is taking place, and understand that the best way to go forward is to listen to the parties who are actually on the ground. And that's hard to do."

In Obama's speech, delivered in Washington just after the president spoke, the Illinois senator vowed to end the war in Iraq sooner rather than later.

"Iraq is not going to be a perfect place, and we don't have unlimited resources to try to make it one," Obama said.

"I will give our military a new mission on my first day in office: ending this war," Obama said, ahead of an expected visit to Iraq and Afghanistan soon, and talks with the leaders of European powers, Israel and Jordan.

Obama's Republican opponent, Senator John McCain argues that Obama would squander gains from last year's troop surge strategy by withdrawing troops, and accuses the Democrat of having a closed mind on Iraq strategy.

But Obama, who also promised to use diplomacy to end Iran's nuclear program and renew US alliances, promised to switch the focus of US military action overseas from Iraq, to Afghanistan and Pakistan.

Baghdad and Washington have been working on an agreement that would set the rules for US soldiers in the war-torn country after the UN mandate for their presence expires at year's end.

Bush had set a July 31 goal for wrapping up the accord -- but negotiations appear to have run into trouble, particularly over whether US troops would be immune from Iraqi prosecution, and whether to set a withdrawal timetable.