Helmand province, where military operations to oust Taleban militants have long been under British control, will fall under the command of US Maj. Gen. Richard Mills, who will direct some 8,000 of Britain's 9,500 troops in Afghanistan.
Maj. Gen. Gordon Messenger, a British military spokesman, denied the change meant handing control of Helmand to the United States, adding that senior British officers would be based at Helmand headquarters.
"That is simply not the case. This is us doing exactly the same job as we've been doing up to now under slightly different arrangements," he told a briefing. "The level of influence and presence that the British have... is considerable and will continue to be so."
The change is part of NATO's plan to split the regional command for southern Afghanistan into two — Regional Command South, and Regional Command Southwest which includes Helmand province, the scene of some of the fiercest fighting between US and NATO forces and a resurgent Taleban.
Britain has the second largest number of foreign troops in the Afghan war after the United States, and has long been America's staunchest ally in military missions, including Iraq.
Some 285 British soldiers have died in Afghanistan since 2001, when the NATO mission there was launched.
New British Prime Minister David Cameron, speaking to reporters in Berlin, was unruffled by the change in command.
"British and American troops have been working together in many theaters over many decades and don't have a problem working together. (They) actually work together extremely well and I know they will in southern Afghanistan as well," he said.
Splitting the southern command in Afghanistan is meant to focus NATO efforts more effectively, given the number of people in each region and its terrain, Messenger said. The change is expected to take effect later in the summer.
Washington is sending more troops to Afghanistan to seize insurgent-held areas before a planned withdrawal starting in July 2011.
Canada plans to withdraw its 2,800 troops from the southern city of Kandahar by the end of next year. Britain has not said when it hopes to pull out its troops.
