"I'm sure I'll have more information and continue to refine my policy," he told reporters in North Dakota.

During his presidential campaign, the Democrat has gone from the hard-edged opponent to Iraq that defined his early candidacy to calling for a phased-out withdrawal of all combat brigades that, at a rate of one or two a month, could last 16 months.

He has said that if al Qaida builds bases in Iraq, he would keep troops either in the country or the region to carry out "targeted strikes".

Responding, Republican National Committee spokesman Alex Conant said: "There appears to be no issue that Barack Obama is not willing to reverse himself on for the sake of political expedience.

"Obama's Iraq problem undermines the central premise of his candidacy and shows him to be a typical politician."

But Mr Obama insisted that his position has not changed at all.

He said he is saying now what he always has - the war was a mistake and needs to be brought to "a responsible end," but "we need to be as careful getting out as we were careless getting in."

This means, he said, that his 16-month timeline was always premised on troops being safe and Iraq being stable.

"I'm going to continue to gather information to see whether those conditions still hold," he said. "My goal is to end this conflict as soon as possible."