All the victims of the assault on World Vision, a major international humanitarian group, were Pakistanis.
Extremists have killed other people working for foreign aid groups in Pakistan and issued statements saying such organizations were working against Islam, greatly hampering efforts to raise living standards in the desperately poor region. Many groups have 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."
Another spokesman said the group had suspended operations across Pakistan as a result of the attack.
Two women were among the six dead, said local police chief Sajid Khan. Four people were wounded.
He said about 10 gunmen took part in the raid.
"They went inside, opened fire indiscriminately and then threw grenades before fleeing," said Khan.
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.
In 2008, militants there killed four Pakistanis working for Plan International, a British-based charity that mainly helps children.
World Vision is one of the world's largest and well-funded Christian aid groups. It was founded 60 years ago in the United States.
