The top U.S. military officer says any agreement to keep American troops in Iraq beyond the end of this year must include guarantees of legal immunity for U.S. forces.

Joint Chiefs of Staff head Admiral Mike Mullen said Tuesday in Baghdad that he understands there are difficult political challenges associated with a potential deal, but reiterated that Iraqi leaders need to decide “as soon as possible.”

Mullen said 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.

Iraqi leaders are expected to discuss the issue on Tuesday.

U.S. leaders have grown increasingly frustrated with the lack of a decision about U.S. forces remaining past a December pullout date.

Both Iraqi and U.S. officials have expressed concern about Baghdad's ability to cope with security after the withdrawal.