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

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.

The attack took place in Ogi, a small town in Mansehra district, which was badly hit by the 2005 Kashmir earthquake.

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

Islamists often target Christian groups, which they accuse of trying to convert Muslims.

Another World Vision spokesman said the group, which was founded 60 years ago in the U.S. and is one of the world's largest and most well-funded Christian aid organizations, had suspended operations across Pakistan as a result of the attack.

About 10 gunmen carried out the assault, which also wounded four people, said local police chief Sajid Khan. Two women were among the six dead, he said.

Another local police official, Liaquat Shah, said the attackers first opened fire inside the office and then left a homemade bomb they detonated by remote control.

"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.

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 lies outside the tribal belt next to Afghanistan where the militants have their main bases and is relatively peacefully.

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, which killed about 80,000 people and left 3 million people homeless.

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 co-ordinator in Islamabad who went to Mansehra today to study the situation," said Salmela.

UN 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 UN operations in those areas, but we definitely condemn any such attack," said Rizvi.

The Pakistani government has fought back against militants staging attacks in the country by launching several military operations over the last year and a half to deprive the insurgents of their sanctuaries in the northwest.

The U.S. has also pummeled militants in Pakistan's tribal area with dozens of deadly missile strikes, including one Wednesday that killed at least four people in North Waziristan, said officials.

Drones fired four missiles at a house and a nearby truck in the Mazer Meda Khel area, some 40 kilometres west of the main town of Miran Shah, said local government official Sabir Khan.

The identities of those killed in the attack 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.