KABUL <http://www.iht.com/cgi-bin/search.cgi?query=KABUL&amp;sort=swishrank> - NATO began Thursday what a representative called its most challenging operation to date, with Britain taking command of the NATO peacekeeping force in Afghanistan in preparation for its expansion into the turbulent south and southeast of Afghanistan over the coming months

In a short ceremony, the new British commander, Lieutenant General David Richards, assumed command from the departing Italian commander and, in a warning to insurgents, promised to build a strong and unified security assistance force bringing NATO and American coalition forces together under one command.

Richards took command immediately of 9,000 troops of the NATO-led International Security Assistance Force, which is stationed in Kabul and the north and west of the country and is conducting primarily reconstruction activities. By the end of July, he will assume command of NATO and other forces in southern Afghanistan, adding combat operations against insurgents to NATO's mission of stabilization and security.

The arrival of 6,000 NATO troops in the south will allow the United States to reduce its force of 19,000 by 2,000 to 3,000 in August. American forces will remain in the border provinces of eastern Afghanistan, and are expected to come under the NATO flag by November, giving Richards command of the entire international military force across the country.

"It will be NATO's most challenging ground operation ever, conducted in parallel with overall change in NATO," Hikmet Cetin, NATO's civilian representative in Afghanistan, said at a news briefing after the ceremony. "NATO cannot afford to fail in Afghanistan, for the whole world and the whole region."

He added: "As we know, security in the south and southeast is still borderline. NATO will be challenged, but as NATO is ready for this challenge, it will not be discouraged. We will do what is needed for success." Afghanistan is the alliance's top priority mission and it is sending its elite force, the Allied Rapid Reaction Corps, to take on the task, he said.

The departure of American troops from southern Afghanistan has already raised concerns among Afghans in the area as they face an increasingly violent insurgency.

Richards, who has served as commander of British peacekeeping forces in East Timor and twice in Sierra Leone, was quick to try to dispel concerns. "I am more than confident the skeptics will be proved wrong," he said.

The number of foreign troops in southern Afghanistan is going to double with NATO coming in, they will have more Apache helicopters than before and American aircraft will remain in support of NATO troops, he said. "The U.S. will remain the major share-holder in the operation, with all the benefits that brings," he said.

He promised to be an "implacable opponent" and make robust use of military force against those who continued to oppose the Afghan government violently. NATO forces would not be directly involved in poppy eradication but would assist in providing the secure environment for Afghan security forces to conduct eradication, he said.