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