Pakistani authorities said about a dozen masked militants stormed the building at about 9:30 a.m., pulled the staff members out of their offices and herded them into a single room, then started shooting indiscriminately. The attackers set off a bomb as they left, destroying the offices, which are located in North-West Frontier Province, in the village of Oghi, near the disputed region of Kashmir.

A spokesman for World Vision said the attackers robbed the employees and stole computers and telephones from the office before opening fire.

Officials in Pakistan said the assailants were of different ethnicities and spoke a mix of Urdu, Pashto and the Pakistani dialect Hindko as they shouted at the aid workers, telling them that they had been “forewarned to stop spreading immodesty.”

World Vision, based on the West Coast, called the attack the worst single act of violence in its 60-year history and said it was suspending its operations in Pakistan as the authorities there investigated the attack. The group has about 180 employees in the country, the vast majority of them Pakistanis.

“There was no advance warning, no threat of violence, nothing,” said Dean Owen, a spokesman for World Vision at its offices in Seattle. “It was unannounced, unprovoked, completely out of the blue.”

There was no immediate claim of responsibility.

World Vision is among the largest Christian charities in the world, and it says it has thousands of staff members working in almost 100 countries on projects related to disaster relief, development, health and other issues. It is among many international aid groups that have had operations in northern Pakistan since October 2005, when a devastating earthquake killed thousands.

World Vision’s office in the area focused on providing relief in the aftermath of the quake, and had been doing community development work, Mr. Owen said.

Militants have stepped up attacks against military, police and civilian targets in North-West Frontier Province since the Pakistani Army began an offensive against them last autumn. On Feb. 20, a police officer was killed in an attack on police stations in Mansehra and Balakot, near the site of Wednesday’s attack.

In February 2008, militants attacked the offices of Plan International, a British aid agency in Mansehra, killing three Pakistani workers.

Government officials in the provincial capital, Peshawar, worry that militants uprooted by the Pakistani Army’s offensive are moving into the northern districts of North-West Frontier Province, which borders Afghanistan, posing new security challenges for the government. There have been reports that militants were moving into tribal areas in the region.

 

Jack Healy contributed reporting from New York.