Thousands of civilians are fleeing the latest military operation against insurgents in Pakistan's northwestern Khyber tribal region, a government official said Sunday.

Pakistan is under intense U.S. pressure to crack down on insurgents along its border with Afghanistan, especially the lawless tribal belt where Al Qaeda leader Usama bin Laden is suspected to be hiding. The U.S. believes militants use Pakistan's tribal areas as safe havens from which to plan attacks on Western troops across the frontier.

Khyber is of particular concern because militants frequently attack trucks along the famed Khyber Pass, a main route for supplies destined for U.S. troops in Afghanistan.

The military destroyed two training centers and 15 militant homes on Sunday, paramilitary troops said. One militant was killed and nine more taken into custody, a written statement from the paramilitary force said, adding that two people kidnapped by militants had also been recovered.

The region is largely off-limits to journalists, making it difficult to verify the information independently.

Farooq Khan, a government official in Khyber, said hundreds of families had been fleeing the region since authorities relaxed a curfew on Friday.

"A few thousand, I think," he told The Associated Press by phone when asked how many civilians have so far fled.

Khan said there were no procedures in place to register the fleeing civilians, making impossible to reach an exact number. He said there were no plans for refugee camps and security forces were "keeping a strict eye" out for any militants trying to blend in.

Three villages in Khyber have been hard hit by the operation. While some families in Malik Din Khel, Sipah and Kambar Khel have left for other villages in the tribal area, most appeared to be leaving Khyber altogether, Khan said. Most of those families were heading to Peshawar, the main city in the northwest, and its neighboring villages.

There have been several army operations in past in the Khyber region that have always concluded with announcements by authorities that the area was cleared of all militants.

The Taliban-affiliated group Lashkar-e-Islam has been a main target of the latest offensive, which authorities say has killed about 90 alleged militants. The operation was launched a week ago after a suicide bombing at a border checkpoint killed 19 police.

Elsewhere, three policemen were fatally shot — each by a single bullet to the head — west of Pakistan's capital.

The dead policemen were discovered early Sunday in their guard room along a railway bridge in Hasan Abdal, a town 25 miles west of Islamabad, police official Arshad Mahmood said. It appeared to be a targeted killing, he said, but he would not say if Taliban militants were suspected.

Pakistani Taliban militants frequently target police, though usually in the northwest.