Defense Secretary Robert Gates said Thursday he is comfortable with Britain's decision to halve its troop levels in Iraq by spring, saying the plan had been worked out jointly with U.S. commanders.

Asked how he would feel about Britain — the Bush administration's staunchest ally in Iraq — cutting even further in the second half of 2008, Gates would say only that it was too early to assess what troop levels would be required beyond next summer, when a U.S. troop reduction in Iraq is to be completed.

The United States has about 165,000 troops in Iraq and plans to pull out at least 21,500 by July.

Prime Minister Gordon Brown announced on Oct. 8 that Britain plans to reduce its forces in southern Iraq from about 5,000 to about 2,500 by spring.

"The reduction of British forces in Basra was based on an assessment of the readiness of Iraqi security forces in the area and was closely coordinated with Gen. Petraeus," Gates said, referring to David Petraeus, the top U.S. commander in Iraq, who has publicly said he supported British troops cuts.

Gates arrived in London on an overnight flight from Washington. He met with Brown at Lancaster House, an aristocratic town house overlooking St. James's Park, and later was meeting with Brown at his offices before flying to Moscow.

The size of the British force in Iraq peaked at about 46,000 at the start of the war and fell to 18,000 within two months.

At a joint news conference with Brown, Gates was asked about a New York Times report that the Marine Corps is pressing to remove its forces from Iraq and to send Marines instead to Afghanistan, to take over the leading role in combat there. That would leave the Army to handle Iraq.

"I had heard that they were beginning to think about that, and that's all that I've heard," Gates said. "I've seen no plan. No one has come to me with any proposals about it. My understanding is that it's, at this point, extremely preliminary thinking on the part of perhaps the staff in the Marine Corps. But I don't think at this point it has any stature."

Gates also reiterated his opposition to a resolution in Congress describing the World War I-era mass killings of Armenians as genocide. Gates said the measure could hurt U.S.-Turkish relations at a time when U.S. forces in Iraq rely heavily on Turkish permission to use their airspace for U.S. air cargo flights.

Britain's participation in the U.S.-led invasion — and the continuing presence of troops in the country more than four years later — remains deeply unpopular among the British public. A total of 170 British soldiers have died in Iraq since the March 2003 invasion.

It was Gates' second visit to the British capital since he replaced Donald H. Rumsfeld as Pentagon chief in December. He stopped here to meet Brown in January before traveling to southern Iraq to consult with British commanders on the situation in Basra. At that point there were about 7,000 British troops there.