The agency said no threats were received ahead of the attack
Militants have attacked the office of a Western aid agency in north-western Pakistan, killing six people, police and the agency have said.
The victims, including two women, were all Pakistani nationals working for World Vision in Mansehra district.
The Christian group said it would suspend all operations in the country.
No group has admitted carrying out the attack but Islamist militants and specifically the Taliban will be suspected, a BBC correspondent says.
Earlier in the week, following a bombing in Lahore, the Taliban said they would carry out more attacks across the country as long as US air strikes and Pakistani army operations against them continued in the tribal areas, the BBC's Aleem Maqbool in Islamabad says.
World Vision said in a statement that it had not received any threatening letters ahead of the attack.
Hundreds of people have been killed in attacks by unmanned drone aircraft in Pakistan's tribal areas since mid-2008.
Two suspected US missile strikes near Miranshah in North Waziristan on Wednesday killed at least 12 people, some of them thought to be foreigners, local officials said.
Gunmen burst into the building in the village of Oghi, some 80km (50 miles) north of Islamabad, early on Wednesday morning.
"About 10 men came, they were all wearing masks. They kicked the doors down, took everyone out of their offices, put them in one place and started shooting," an office administrator told the AFP news agency.
He added that a bomb had been thrown as the gunmen left the building after the attack, leaving a large crater by the door.
According to a local police officer quoted by the Associated Press, the bomb was a detonated by remote control after the attackers had left the building.
"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," said Liaquat Shah.
World Vision confirmed that seven members of staff had been injured during the attack.
"We are deeply sorry we've lost staff members, who were locals, who were deeply committed to improving lives in Pakistan," World Vision's spokesman James East told the Associated Press news agency.
Attacks on aid workers and other non-governmental organisations (NGOs) are not uncommon in Pakistan.
In February 2008, British aid agency Plan International suspended its operations there after three of its workers were killed in an attack on its office in Mansehra.
Correspondents say Mansehra, in North West Frontier Province (NWFP), had served as a base for militants operating in Afghanistan and Kashmir.
Last month two simultaneous suicide attacks on police targets in Mansehra left one police officer dead and several other people injured.
Militant groups and religious parties in Pakistan have often accused NGOs of spreading "vulgarity" in society.
This is because NGOs mostly employ women workers and organise mixed social gatherings in line with their professed policy of gender equality, observers say.