US:The three leading Democratic presidential candidates have said that they cannot guarantee that all United States troops will have left Iraq by the end of the next presidential term in 2013.
At a debate in Dartmouth, New Hampshire, Hillary Clinton, Barack Obama and John Edwards said that some US forces may have to remain after combat operations wind down to protect the US embassy in Baghdad and fight al-Qaeda in Iraq.
"I think it's hard to project four years from now, and I think it would be irresponsible. We don't know what contingency will be out there," Mr Obama said.
Mrs Clinton said that although she would hope to withdraw all troops by the end of her first term, she could not promise to do so as "it is very difficult to know what we're going to be inheriting".
Mr Edwards said he would immediately reduce the 100,000 troops expected to be in Iraq in 2009 to between 40,000 and 50,000, but said that up to 5,000 would have to stay indefinitely to protect US civilian personnel.
The debate came as polls showed Mrs Clinton maintaining a big lead over Mr Obama in national polls, with Mr Edwards trailing far behind in third place. Mrs Clinton also enjoys a comfortable lead in early voting states, although the three leading candidates are running almost level in Iowa, which holds the first caucus in January.
Mr Edwards sought to differentiate himself sharply from Mrs Clinton on domestic and foreign policy issues, warning that her vote this week in support of designating Iran's Revolutionary Guard as a terrorist organisation risked giving President George Bush an excuse to attack Iran.
"I voted for this war in Iraq and I was wrong to vote for this war and I accept responsibility for that. Senator Clinton also voted for this war," Mr Edwards said.
"We learned a very different lesson from that . . . Because what I learned in my vote on Iraq was, you cannot give this president the authority and you can't even give him the first step in that authority, because he cannot be trusted."
Mrs Clinton was more defensive than in earlier debates, sometimes flatly refusing to answer questions posed by NBC's Tim Russert.
However, she made no obvious mistakes that could damage her campaign. Mr Obama, who was suffering from a cold, failed to make much impact, leaving it to Mr Edwards to take the lead in attacking the frontrunner.
Meanwhile, top Republican candidates faced sharp criticism from African-American political activists yesterday for their failure to take part in a debate on minority issues at Morgan State University in Baltimore last night.
Rudy Giuliani, Mitt Romney, John McCain and Fred Thompson have all pleaded scheduling conflicts but critics pointed out that they also turned down an invitation to take part in a debate on a Hispanic television channel.
All the Democratic candidates have taken part in debates aimed at the African-American and Hispanic communities and most participated in a televised forum on gay issues.