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ISLAMABAD, Pakistan - Drone aircraft unleashed two missile attacks in a lawless tribal region on the Afghan border Tuesday, making September the most intense period of U.S. strikes in Pakistan since they began in 2004, intelligence officials said.

The stepped-up campaign 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.

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 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. 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.

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's becomes an intel feedback loop," that fuels further strikes, he said.

U.S. officials do not publicly acknowledge the missile strikes but have said privately that they have killed several senior Taliban and al-Qaida militants and scores of foot soldiers in a region largely out of the control of the Pakistani state.

Critics say innocents are also killed, fueling support for the insurgency.

A Pakistani intelligence official told the AP that "most of the fighters killed in recent weeks are from the Haqqani network," adding that Arab militants had also been killed. He spoke on condition of anonymity because of the sensitivity of the topic.

"We live in constant fear," said Munawar Khan, 28, who lives in the nearby village of Darpa Khel. "We have missile strikes every day."

U.S. forces began targeting Pakistan's tribal regions with aerial drones in 2004 but the number of strikes soared in 2008 and has been steadily climbing since then, with nearly 70 attacks this year, according to an AP tally.

There has been little evident public or official outrage inside Pakistan in the wake of September's airstrikes, but the Pakistani government says it has not altered its long-standing objection to such attacks, which have also targeted Pakistani Taliban militants who carry out attacks inside the country.

"The position of the army and government is the same, that it harms more than it helps," said Maj. Gen. Athar Abbas, an army spokesman.

The Haqqanis worked closely with Pakistan's intelligence service during the anti-Soviet war and have not waged attacks inside Pakistan.

In Afghanistan, however, they often use suicide bombs in civilian areas and do not let suicide bombers back out of an attack, unlike the Afghan Taliban, the U.S. intelligence official told the AP.

There's some disagreement in U.S. intelligence ranks as to whether the Haqqanis are part of the Taliban, or simply allied with them in what an intelligence official in the U.S. called "a marriage of convenience."

Many in the Haqqani leadership have roles as Taliban commanders. But officials say the Haqqanis seek dominion only over the areas in which they hold sway -- Afghanistan's mountainous eastern provinces of Paktika, Paktia, and Khost, stretching to the outskirts of the capital, Kabul. The Taliban, by contrast, want to take over the whole country. The two ruled those areas side by side when the Taliban governed Afghanistan -- though Haqqani was subservient to Taliban ruler Mullah Omar and did not have independence.