Iraq's political leaders have agreed to allow the government to negotiate a possible deal with the U.S. that would keep American troops in the country on a training mission lasting beyond the end of 2011, when they are due to withdraw.

Foreign Minister Hoshyar Zebari Tuesday said a final agreement is far from settled, and that no details have yet emerged about how many U.S. troops could stay if a deal is reached.

Earlier Tuesday, U.S. Joint Chiefs of Staff head Admiral Mike Mullen said he understands there are difficult political challenges associated with a potential agreement, but he repeated that Iraqi leaders need to decide “as soon as possible.”

Mullen told reporters there will come a time when it is too late to reverse a planned withdrawal, and all of the U.S. troops will have to leave. He also said any agreement to keep American troops in Iraq beyond the deadline must include guarantees of legal immunity for U.S. forces.

The joint chiefs chairman commented after a meeting in Baghdad with Prime Minister Nouri al-Maliki. He also accused neighboring Iran of interfering in Iraq. He said Tehran has been arming militants who carry out attacks on Iraqi soil.

U.S. forces are scheduled to withdraw from Iraq at the end of the year, but both U.S. and Iraqi officials have expressed concern about Baghdad's ability to cope with security after the withdrawal.

The United States has grown increasingly frustrated with the lack of a decision from Iraq on whether it will seek an extension.