WASHINGTON (Reuters) - The Pentagon said on Friday that it has scrapped plans to deploy a 3,500-member Army brigade to Iraq in January, accelerating a U.S. drawdown as the White House weighs sending more troop to Afghanistan. The move, approved by Defense Secretary Robert Gates, means that when an existing unit in Iraq pulls out in January, it will not be replaced, Pentagon spokesman Bryan Whitman said. The decision means the brigade could be sent later to Afghanistan, but Whitman said that was not taken into account. The decision was based purely on the improved security environment and the continued improvement in the ability of Iraqi security forces, and is completely unrelated to our operations in Afghanistan, Whitman said. The 3,500-member brigade whose deployment to Iraq was canceled has yet to be given a new assignment, Whitman said. They return to the pool of available resources, he added, refusing to say whether the unit could now be sent to Afghanistan as part of a proposed buildup there. The U.S. and NATO commander in Afghanistan, General Stanley McChrystal, has recommended sending 40,000 additional troops to Afghanistan. More than 100,000 are already there, including 67,000 U.S. forces. President Barack Obama is considering the request as part of a broad review of U.S. war strategy. Officials say he has yet to make a decision. The commander of U.S. forces in Iraq, General Ray Odierno, said earlier this month that he expected the number of U.S. troops in Iraq to drop to as few as 110,000 by the end of this year. There currently are about 119,000. The U.S. combat mission in Iraq is slated to end on Aug. 31, 2010. Provided the situation at that time is peaceful, Odierno said, the United States would then draw down to a 50,000-member transition force. That force would train and equip Iraqi forces, and protect provincial reconstruction teams, international projects and diplomatic staff.