WASHINGTON (Reuters) - The United States could begin withdrawing troops from northern Iraq in January, the commander of U.S. forces for the region said on Friday, as pressure mounts on the Bush administration to end the war.

Army Maj. Gen. Benjamin Mixon said a drawdown could take place over 12 to 18 months if U.S. and Iraqi forces continued to make progress in establishing security in his area. But he cautioned against any reduction of forces in northern Iraq this year. "I think that over time in a very methodical and well thought-out way -- and I'm only speaking for Multi-national Division North -- that we could have a reduction of force that could begin in January of 2008," Mixon said. Mixon's area of responsibility includes Iraqi cities such as Kirkuk, Tikrit and Mosul as well as the volatile province of Diyala, the scene of a major U.S. military operation in recent weeks against the militant group al Qaeda in Iraq. Speaking by videolink from Iraq to reporters at the Pentagon, Mixon said he had about five or six brigades under his command and that number could be cut in half over 18 months if security improved and Iraqi forces became more capable. A brigade normally has around 3,000 to 5,000 troops. As Iraqi forces took over more security duties, U.S. forces would concentrate on providing logistical support, air power and medical assistance, Mixon said. President George W. Bush is under pressure from lawmakers in his own Republican Party, Democrats and U.S. public opinion to pull troops out of Iraq. He says he wants to start a drawdown as security conditions allow and as his military commanders in Iraq advise, but has refused to give specific dates. Despite a veto threat by Bush, the Democratic-controlled House of Representatives voted 223-201 on Thursday to approve legislation to bring combat troops out of Iraq by April 1, 2008. Bush has urged Congress to wait until a progress report due in September before considering any change of course. Mixon said a drawdown in his area should not be based simply on a feeling that it was time to get out. "It needs to be well-thought out and it cannot be a strategy that is based on 'Well, we need to leave.' That's not a strategy, that's a withdrawal," he said. "It seems to me that we should first decide what we want the end-state to be in Iraq and how is that end-state is important to the United States of America, to this region and to the world and then determine how we can reach that end-state and how much time that will take," he said. "To me, that seems to be the most important thing, because there will be consequences of a rapid withdrawal from Iraq.