The stepped-up campaign -- which included two missile attacks Tuesday in a tribal region on the Afghan border -- is focused on an area of farming villages and mountainous, forested terrain controlled by Haqqani militants, U.S. officials say.

U.S. officials said the airstrikes were designed to degrade the Haqqanis' operations on the Pakistani side of the border, creating a "hammer-and-anvil" effect as U.S. special operations forces carry out raids against their fighters across the frontier in Afghanistan. The officials spoke on condition of anonymity to discuss ongoing classified operations.

The missiles have killed more than 50 people in 12 strikes since Sept. 2 in the Pakistani region of North Waziristan, according to an Associated Press tally based on Pakistani intelligence officials' reports.

U.S. officials did not discuss specific reasons for the surge of airstrikes this month. Until now, the highest number of airstrikes inside Pakistan in a single month had been the 11 launched last January after a suicide bomber killed a Jordanian intelligence officer and seven CIA employees at a base in Afghanistan.

U.S. and Pakistani intelligence officials said most of this month's strikes have targeted the forces of Jalaluddin and Sirajuddin Haqqani, a former anti-Soviet commander and his son who are now battling U.S. forces in eastern Afghanistan.

The raids targeting the group in Afghanistan are led mainly by the Joint Special Operations Command, and they have also been stepped up.

A senior U.S. intelligence official in Afghanistan said the United States had reports that Haqqani commanders were under pressure from the operations. "We're seeing from some of the raids that some of the more senior guys are trying to move back into Pakistan," the official said, speaking on condition of anonymity to discuss matters of intelligence.

The official cautioned that so far, neither the special operations raids nor the missile strikes on the Pakistani side of the border appear to have degraded the militants' ability to fill their ranks.

The Pakistan army has launched several offensives in the tribal regions over the last 2 1/2 years but has not moved in force into North Waziristan. The United States is unable to send ground forces into Pakistan and must rely on the drone strikes.