The troops deployed four days ago by air and by land into the region — a "very rough terrain" of peaks between 8,000-13,000 feet high, British Marine Spokesman Lt. Col. Paul Harradine said.
The officials would not say exactly where the operation was taking place, but it appeared to open up a new front in the search for fighters from Osama bin Laden's al Qaeda movement.
In recent weeks, that search has concentrated on an area straddling the border just southeast of Kabul, around the Afghan towns of Khost and Gardez and the Pakistani region of Waziristan. Allied troops have been sweeping through villages in the region, trying to track down small groups of al Qaeda or Taliban fighters, and there have been unconfirmed reports of top al Qaeda leaders in the border area.
But Harradine said the latest operation, called Operation Snipe, was not taking place in the Khost-Gardez region or near the Pakistani border.
The force of 1,000 troops, "equipped with the full range of combat power," has a mission to secure and search a large area "in what is a strategic key location for our enemy," Brig. Roger Lane told reporters at Bagram air base.
Lane said the new region was "one of the few remaining areas in Afghanistan that has never before been investigated by coalition ground forces. We have good reason to believe that it is, or has been, a key base for the al Qaeda terrorist network."
U.S. air power and U.S. special operations troops will support the British-led operation, U.S. military spokesman Maj. Bryan Hilferty said. Lane said Afghan ground troops were also involved.
Earlier, Pentagon officials in Washington spoke of a buildup of multinational forces along the border with Pakistan — raising the possibility of a new major thrust against remaining al Qaeda and Taliban fighters.
The officials said several hundred Afghans, Australians, British and soldiers from the United States' 101st Airborne Division were deploying for missions aimed at finding enemy fighters.
Other recent developments include: