"I do think it is important that this be conditions-based," said Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, at a Pentagon news conference.

    The US top military officer made remarks after the country has signed a security agreement with Iraq, known as the Status of Forces Agreement (SOFA), that would require American forces to leave by the end of 2011 without any details disclosed yet.

    After the agreement was sealed, top US commanders in Iraq have said that they all believe Iraqi forces would be ready to defend themselves in the next three years, said Mullen.

    "I would say if that improvement would continue at the pace we see right now, that they will be able to do that," he added.

    However, Mullen insisted "conditions could change in that period of time (three years)," and the United States would continue to talk with Baghdad "as conditions continue to evolve."

    Asked if the agreement is likely to be subjected to change, he said "that's theoretically possible."

    The remarks run against President-elect Barack Obama's promise that bring all US combat troops from Iraq with responsibility within 16 months after he takes office by mid-2010.

    But Mullen said that he would follow whatever Obama's directions after he is sworn in on Jan. 20, and give him best military advice on Iraq, among others.

    "Should president-elect Obama give me direction, I would carry that out," he said. "I mean, that's what I do as a senior member of the military."

    Currently, there are about 150,000 US troops in Iraq.