ISLAMABAD: Suspected U.S. missiles killed at least 12 alleged militants Thursday in a compound formerly used as a religious school in Pakistan's North Waziristan tribal region, officials said, the eighth such attack in two weeks.

The strike illustrated the Obama administration's unwillingness to abandon its missile campaign against insurgent targets along Pakistan's northwest border with Afghanistan. Despite Pakistani protest, the attacks have surged in number in recent days.

Nearly all the attacks in recent months have focused on North Waziristan, a segment of Pakistan's semiautonomous tribal belt where militant networks focused on battling the U.S. and NATO in Afghanistan are based. Some of those militants are believed to have been involved in a late December attack that killed seven CIA employees in eastern Afghanistan.

It's a region that the Pakistani military has been wary of treading, partly because groups based there have not directly threatened the Pakistani state. The army has struck truces with some of them to keep them out of its battle against the Pakistani Taliban — who have attacked Pakistan in numerous ways — in neighboring South Waziristan.

At least two missiles hit the Pasalkot area of North Waziristan around 7 a.m., landing in a sprawling compound that has been used as a religious school in the past. The identities of the dead were not immediately known, an army official and an intelligence official said.

The officials spoke on condition of anonymity because they were not authorized to speak to media on the record.

The strike came as Richard Holbrooke, a U.S. special envoy to Pakistan and Afghanistan, was visiting parts of Pakistan.

The U.S. rarely discusses the covert missile campaign, though in the past American officials have lauded it as a successful tactic that has killed several top al-Qaida operatives as well as Pakistani Taliban chief Baitullah Mehsud.

Pakistan formally protests the drone-fired strikes, saying they violate its sovereignty and spur more anti-American sentiment among the population, but many analysts believe the nuclear-armed South Asian nation secretly aids the campaign.

During a Wednesday media conference with Holbrooke, Pakistani Foreign Minister Shah Mahmood Qureshi stopped short of completely ruling out the missile attacks, but said there were certain "red lines" that Washington must not cross.

"Pakistan feels that it would undermine our relationship if there is expansion of drones and if there are (U.S.) operations on the ground," Qureshi said.