The Shawal valley on the border in Pakistan's North Waziristan tribal area has long been a no-go area for Pakistani troops, where Pashtun tribes have ruled without interference for generations and fugitives have long found refuge.

Now, Pakistani forces, as part of their campaign to clear foreign militants from the swathe of tribal lands along the border, seem to have set their sights on Shawal.

"There has been some speculation, it's one of the places mentioned as a possible hideout," said Pakistani Afghan expert and journalist Rahimullah Yusufzai, referring to the hunt for bin Laden and his deputies.

But Yusufzai and other analysts doubt bin Laden would stay there if he knew a sweep was being mounted.

The Shawal is an upland valley, with forests and meadows where tribesmen graze their flocks in summer, at about 1,300 metres (4,000 feet), criss-crossed by ravines and ridges soaring up to 3,400 metres (11,000 feet).

The small natural fortress, seldom visited by outsiders, lies on both sides of the unmarked border.

Pakistan sent troops into its side of the area for the first time in June 2002. The Pakistani military has given no details of its current plans but officials say troops are moving towards Shawal.

"This area has not really been controlled by the government. They would like to have a permanent presence there but it is going to be difficult," Yusufzai said.

"There are forests, natural hideouts in the mountains and it's very remote. People there have been allowed to live on their own with no government control for so long, establishing government writ, therefore will be very difficult."