Suspected militants armed with assault rifles and a homemade bomb attacked the offices of a U.S.-based Christian aid group helping earthquake survivors in northwestern Pakistan on Wednesday, killing six Pakistani employees, police and the organization said.

The attack prompted World Vision, a major international humanitarian group, to suspend its operations in Pakistan. Other aid organizations condemned the violence but said it would not lead them to curtail their own activities.

The assault took place in Ogi, a small town in Mansehra district that was badly hit by the 2005 Kashmir earthquake, which killed about 80,000 people and left 3 million people homeless.

"It was a brutal and senseless attack," said Dean Owen, World Vision spokesman in Seattle, Washington. "It was completely unexpected, unannounced and unprovoked."

Extremists have killed other foreign aid group employees in Pakistan and accused such organizations of working against Islam, greatly hampering efforts to raise living standards in the desperately poor region. Many groups have already scaled down operations in the northwest or pulled out altogether.

Wednesday's attack may have been prompted by World Vision's religious affiliation. Islamists often target Christian groups, which they accuse of trying to convert Muslims.

World Vision was founded 60 years ago in the U.S. and is one of the world's largest and most well-funded Christian aid organizations.

The Pakistani government has fought back against militants in the northwest with a series of military operations, and the U.S. has pummeled the insurgents with dozens of drone strikes near the Afghan border, including two Wednesday that killed at least 15 people.

The 10 gunmen who attacked the World Vision office first opened fire and then left a homemade bomb they detonated by remote control, police official Liaquat Shah said.

"They left a locally made pressure cooker bomb that exploded soon after the attackers fled the scene, killing NGO people first by gunfire and then with the blast," Shah said.

Two of the six dead were women, and another four people were wounded, said local police chief Sajid Khan.

The attack seriously damaged the aid group's office, leaving the ground strewn with rubble and the concrete walls pockmarked with bullet holes, AP Television News footage showed. A tattered red office chair lay overturned among the debris.

Al Qaeda, the Taliban and allied groups are strong in northwestern Pakistan, but Mansehra has been relatively peaceful and lies outside the tribal belt next to Afghanistan where the militants have their main bases.

Islamist militants see foreign aid groups and local outfits that receive international funds as a challenge to their authority in regions under their influence. The organizations often employ women and support female rights initiatives, further angering the extremists.

Many foreign aid groups set up offices in Mansehra after the 2005 earthquake. In 2008, militants there killed four Pakistanis working for Plan International, a British-based charity that mainly helps children. The attack forced several foreign agencies to scale back assistance to the area.

But aid groups said that Wednesday's attack would not cause them to suspend their operations.

"I would not feel that it means anything but intensifying our own way of doing things," said Pepe Salmela, the country director for the International Federation of Red Cross and Red Crescent Societies, which works on community-based health and disaster preparedness programs in Mansehra.

"We have quite a big security team and a security coordinator in Islamabad who went to Mansehra today to study the situation," said Salmela.

U.N. spokeswoman Ishrat Rizvi also said the attack would not curtail the world body's activities in Pakistan.

"Any single attack on any implementing partner would not result in the closure of U.N. operations in those areas, but we definitely condemn any such attack," Rizvi said.

The suspected U.S. missile strikes that occurred Wednesday hit the Mazer Meda Khel area of the North Waziristan tribal region, said local government official Sabir Khan.

In the first attack, drones fired four missiles at a house and a nearby truck, killing six people, said Khan.

As locals were trying to recover the bodies from the attack, drones fired two more missiles at the group and another nearby vehicle, killing nine people, Khan said.

The identities of those killed in the attacks were not known.

The U.S. refuses to discuss publicly the drone program in Pakistan, but officials say privately it has killed several senior Al Qaeda and Taliban commanders.