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-Qaida 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-Qaida or Taliban fighters, and there have been unconfirmed reports of top al-Qaida 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-Qaida 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-Qaida 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.
On Wednesday, U.S. troops participating in the search on the Pakistani side of the border came under hostile fire for the first time since their operations began in recent days.
A rocket attack in the early morning apparently targeted a building where U.S. special forces were sleeping in the town of Miram Shah, near the border and about 40 miles south of Khost. The rocket missed, striking a hit a building about 300 yards away. No one was hurt.
A local official in Miram Shah, who spoke on condition of anonymity, said the rocket apparently came from the Afghan side of the border.
It wasn't clear who fired the rocket, but local residents found pamphlets in the morning saying Pakistan's rulers had "challenged the faith and Islamic honor ... by bringing American commandos" to the area. The pamphlets were signed by a previously unknown group called Mujahedeen of North Waziristan.
U.S. special forces troops have been working in small groups alongside Pakistani forces in the tribal-run region along the Afghan border — an area that that has been a stronghold of support for bin Laden. How many U.S. soldiers are operating in Pakistan is unknown; both sides have said little about the operations, trying to avoid angering Islamic groups in Pakistan.
British officials would not say what size an al-Qaida force was thought to be in the region being searched by Operation Snipe.
"The success of this operation will not be measured solely in a count of the number of dead terrorists," Lane said. "We will first seek out and destroy all the terrorist infrastructure in this region and, of course, do likewise for any al-Qaida forces that we encounter."
Britain has about 1,700 troops deployed at the Bagram air base north of Kabul. Harradine said that for the new mission, four British fighting companies — each about 120 men — were deployed from Bagram, supported by 105 mm howitzer gun batteries.
U.S.-led forces have been focusing heavily on the eastern part of Afghanistan since Operation Anaconda in the first two weeks of March. That campaign, the biggest ground operation of the war, sought to flush fighters out of an area near Gardez. Since it ended, commanders have continued to send teams and patrols through provinces along the border to find fighters and weapons caches.
In other developments, CBS News Correspondent Lee Cowan reports that American and Afghan troops have landed in a potentially dangerous position.
U.S. officials say that for the first time, Afghanistan's interim leader Hamid Karzai has ordered an attack on a group of his own Afghan forces.
The target is Padshah Kahn, an Afghan warlord who agreed to talk with CBS News in private. He admits he's responsible for a string of attacks on the interim government this month -- but he's also officially a U.S. ally, charged with protecting American special forces in Khost.
That's the same Pakistani border town where allied troops have just come under attack in two separate incidents. The firefights between U.S.-led special forces and suspected al Qaeda terrorists left up to four of the suspects dead, a U.S. general said.