In Operation Buzzard, British forces will conduct patrols in populated and rural areas by helicopter, on foot and in vehicles in a mission that will last longer than previous sweeps in the region, said Lt. Col. Ben Curry, a British spokesman.
Some 300 British troops were deployed over the past three days in the area, but a senior British official said the number could rise to 700.
"The key point is being unpredictable, operating overtly and covertly to introduce doubts in the minds of the al Qaeda and Taliban," Curry said, adding that the operation will last "a couple of weeks."
U.S. Maj. Bryan Hilferty said the mission is "to interdict al Qaeda and Taliban and infiltration routes and safe havens, to deny their freedom of maneuver, deny them sanctuary and gather intelligence."
The operation is being launched at a time when coalition officials warn that al Qaeda and the Taliban may launch insurgency attacks, including suicide bombings, to disrupt the June 10-15 meeting by the loya jirga, or grand council, to choose a transitional government for Afghanistan.
The British troops are operating in the plains south and east of the city of Khost near the Pakistani border, in sharp contrast to the rough mountainous regions where past British sweeps have taken place. The area is a key region for border crossings.
U.S. and Afghan officials say they have no information about specific threats, but there have been recent reports of possible attacks on Kabul, where 1,051 representatives will meet in two weeks to replace the interim government put in place with U.N. backing in December.
In addition, the U.N. said it was "deeply disturbed" at reports that participants in the loya jirga process were being intimidated, threatened and even detained in western Herat province. Eight other Afghans associated with the run-up to the meeting have been killed this month. U.N. officials emphasized they had no direct evidence linking the deaths with the selection process.
Two former high-ranking Taliban officials said last week that the Afghan-Pakistan border cannot be sealed to stop the movement of militants. They said the Taliban's supreme leader, Mullah Mohammed Omar, is overseeing a reorganization of the religious movement and has been in contact with Taliban warriors in their mountain hide-outs in Afghanistan.
In other war-related developments: