About half of the aid groups operating in Mansehra district of northwest Pakistan have pulled out or shut down after gunmen staged a brazen and highly sophisticated attack that killed four local employees of a British aid group, according to aid workers and Pakistani officials. 

No one has claimed responsibility for Monday's incident, said Mazhar Ul Haq Kakakhel, the police chief for Mansehra district. Pro-Taliban militants have been increasingly active in Pakistan's Northwest Frontier Province where the execution-style killings took place.

A dozen attackers wearing fatigues and carrying automatic rifles stormed the offices of Plan International and opened fire on its employees. Then they tossed a bomb into the compound and set the place ablaze. Survivors of the attack said two of the assailants appeared to be older and to be commanding the operation.

Violent attacks on aid groups are common in Afghanistan's troubled provinces. This was the first of its kind in Pakistan.

"The general feeling in the aid community is that this wasn't an isolated event," said Graham Strong, who chairs the Pakistan Humanitarian Forum. "We are worried there will be more."

The 21-group forum said the "outrageous" attack disrupted relief work aimed at helping "vulnerable Pakistani children and their families." Dozens of aid groups moved into Mansehra following the 2005 earthquake that killed more than 73,000 people in the Northwest Frontier Province and in Pakistani-held Kashmir.

Plan International, which has operated in the area for 12 years, provides education assistance, the group said in a statement. The group is still considering whether to continue work there.

Amid rising Taliban violence in the frontier, Western aid groups in Mansehra have been told previously to remove female staff, and co-ed schools have received anonymous threat letters, saying girls must wear burqas to school or face "serious consequences."

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