PESHAWARA U.S. drone strike in Pakistan's northwestern Shawal Valley has killed at least five militants , three Pakistani intelligence sources said today. Monday night's strike in the Zwy-Naray area of North Waziristan, a mountainous region bordering Afghanistan, follows a build up of Pakistani forces in the area and another drone strike on Saturday.

The deep, forested ravines of Shawal Valley are dotted with Taliban bases and are a key smuggling route into Afghanistan. The valley runs across South and North Waziristan, most of which are under the control of the Pakistani military.

The military launched an offensive in North Waziristan in June, but militants and intelligence sources say the Shawal Valley remains largely untouched by the operation.

Last week, residents and military sources said Pakistani forces were moving toward the valley from north and south. But the scale and progress of the operation is unclear.

The area is off limits to journalists. Phone lines to many areas have been cut and roadblocks control civilian movement.

Authorities in Afghanistan say they have not seen an influx of civilians fleeing fighting.

The Pakistani government usually protests drone strikes, complaining they are a violation of sovereignty. On Monday, the government issued a standard press release protesting Saturday's strike.

But many in Pakistan suspect that the military works alongside the United States to guide the strikes. There were no strikes at all during the six months that Pakistani officials pursued peace talks with the Taliban last year.

But the strikes resumed a days before the military launched its offensive in North Waziristan.