Mir Ali, Pakistan ? Drone aircraft unleashed a missile attack in a lawless tribal region on the Afghan border today, keeping up the most intense period of U.S. strikes in Pakistan since they began in 2004, intelligence officials said.

The stepped-up campaign that included today’s strike is focused on a small area of farming villages and mountainous, thickly forested terrain controlled by the Haqqani network, a ruthless American foe in Afghanistan, U.S. officials say. There is some evidence the network is being squeezed as a result, one official said.

In the latest strike, U.S. missiles killed 12 people in a house in Dargah Mandi, 2.5 miles west of the main town of Miran Shah in North Waziristan, Pakistani intelligence officials told The Associated Press.

American 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 60 people in 13 strikes since Sept. 2 in the Pakistani region of North Waziristan, according to an Associated Press tally based on Pakistani intelligence officials’ reports. Many struck around Datta Khel, a town of about 40,000 people that sits on a strategically vital road to the Afghan border.

The border region has long been a refuge for Islamist extremists from around the world. Osama Bin Laden and other al-Qaida leaders are believed to have fled there after the U.S.-led invasion of Afghanistan in 2001.

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 American forces in eastern Afghanistan.

The raids targeting the group in Afghanistan are led mainly by the Joint Special Operations Command. Such raids across Afghanistan are now more frequent than at any previous time in the nearly nine-year war, with some 4,000 recorded between May and August as special operations numbers were boosted by troops arriving from Iraq.

The raids have focused on the Haqqanis for the last two years, officials said.

A senior American intelligence official in Afghanistan said the U.S. 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 the Haqqanis often employ military disinformation. And so far, the official said, neither the special operations raids nor the missile strikes on the Pakistan side of the border appear to have degraded the militants’ ability to fill the ranks of the slain.

But sometimes, the U.S. official said, the replacements are far less competent than their predecessors.

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 U.S. is unable to send ground forces into Pakistani territory, and must rely on the drone strikes.

A major offensive in North Waziristan became even less feasible last month after massive flooding forced tens of thousands of Pakistani soldiers to focus exclusively on rescuing stranded victims, redirecting flood waters and rebuilding damaged infrastructure.

Last month also saw a lull in U.S. airstrikes, until an attack on Sept. 2 began days of repetitive missile attacks.

U.S. officials did not discuss specific reasons for the surge of airstrikes this month. A former American military official said poor weather often hampers drone operations.

Until now, the highest number of airstrikes inside Pakistan in a single month had been the 11 launched in January 2010 after a suicide bomber killed a Jordanian intelligence officer and seven CIA employees at a base in Afghanistan.

“Usually when there’s this type of intensity in strikes, they’re going after something specific,” Bill Roggio, of the Long War Journal, which tracks the strikes, said of this month’s attacks. “They hit it, watch what moves, then hit it again. It becomes an intel feedback loop,” that fuels further strikes, he said.