Iraq's political leaders gave the government the green light Tuesday to begin negotiating a deal with the U.S. to keep American troops in the country past the end of the year to train Iraqi security forces.

But Foreign Minister Hoshyar Zebari said a final agreement is still far from settled, and cautioned that Baghdad could still insist the U.S. military leave by the end of 2011 as required under a 2008 security agreement.

"The government still might not do it (allow U.S. troops to stay)," Zebari told The Associated Press after the closed-door discussions.

"This is a politically highly charged issue, and there was division," he said. "But this meeting unified all the political leaders to back the government and start the negotiations."

The small step forward was the result of five hours of often-heated debate among several dozen Iraqi political leaders and Cabinet ministers. Zebari said no details were settled — like how many U.S. troops would stay, or for how long, or whether they would be given legal immunity from prosecution.

Those issues will be key factors as Washington weighs whether it will continue its military presence in Iraq after more than eight years of war.

Iraq's leaders are torn between the nation's shaky security and its war-weary public in deciding whether U.S. forces should leave by Dec. 31. The issue has also put Prime Minister Nouri al-Maliki in an uncomfortable position with one of his top allies, anti-American cleric Muqtada al-Sadr, who is bent on driving American forces from the country.

Washington has offered to have up to 10,000 U.S. troops stay and continue training Iraqi forces on tanks, fighter jets and other military equipment.

Al-Maliki, who likely will lead the negotiations, has proposed that parliament ultimately could vote on the troop dilemma. That would shield him from political fallout should lawmakers approve asking the U.S. military to stay, although American officials fear there's not enough support in parliament for such a motion to pass.

Earlier Tuesday, U.S. Joint Chiefs chairman Adm. Mike Mullen left Iraq after a brief visit during which he urged the government to quickly request the troops to stay — or face their imminent departure.

"A significant part of this is just a physics problem," Mullen, the outgoing top U.S. military officer, told reporters in Baghdad Tuesday morning. "You get to a point in time where you just can't turn back and all the troops must leave. That's why it's so important to make the decision absolutely as soon as possible."

Mullen also said the troops must be given immunity as part of any agreement, and that it must be approved by Iraq's parliament.

About 46,000 American troops are currently in Iraq.