Khan told the AFP news agency that some armed people had "stormed" the building of the NGO.

"They first set off a bomb and then opened fire. We don't know the exact number of casualties but initial reports suggest four to five people were killed," he said.

Grenades used

Al Jazeera's Hashem Ahelbarra, reporting from Swat, an adjoining region in Pakistan's northwest, quoted police as saying that the assailants opened fire and exploded grenades once inside the building.

"They managed to get inside the offices of [World Vision] and plant explosive devices which were then used to destroy the entire building," he said.

in depth
  The Taliban's influence in Pakistan
  Swat: Pakistan's lost paradise
  Profile: Pakistani Taliban
  Talking to the Taliban
  Pakistan's war

World Vision, a large Christian humanitarian group, is helping survivors of the October 2005 Kashmir earthquake in the area.

"We are deeply sorry we've lost staff members who were locals who were deeply committed to improving lives in Pakistan,'' James East, a World Vision spokesman, said.

Mansehra town has been a hub for relief efforts following the earthquake, which killed 73,000 people.

The area has been generally peaceful although there have been occasional incidents of violence.

In 2008, armed men attacked an office of the Plan International, a British-based charity that mainly helps children, killing four Pakistani staff members.

Mansehra is to the east of Swatwhere the army launched an offensive a year ago to clear out the Pakistani Taliban.

Fears of influx

The deadly assault raised fears that the fighters might be pushed into Mansehra.

"This attack comes against the backdrop of a long history of animosity or anti-NGO sentiment which has been on the rise in Pakistan," our correspondent said.

"Particularly when the Tehreek-e-Taliban Pakistan, (TTP), Pakistan's Taliban, took over major parts of Pakistan, such as the Swat valley, Waziristan, Quetta and tribal belts.

Taliban in Pakistan has often accused NGOs of spreading 'vulgarity' in society

"After the Taliban was driven away from this part of Pakistan [Swat] and forced to take refuge in the mountains, they are now resorting to hit-and-run tactics to show the Pakistani army that they have not been completely undermined."

Fighters have killed other people working for foreign aid groups in Pakistan and issued statements saying such organisations were working against Islam.

The aid groups are seen by the fighters as a challenge to their authority in regions under their influence. This is because the NGOs mostly employ women workers and support female rights initiatives.

"Taliban groups and religious parties are spreading this rhetoric about international NGOs, saying that they are not innocent organisations, that their mission is a cover for a broader attempt by the Americans and the 'infidels', a term they would use all the time to refer to the international community, to weaken Islam," Al Jazeera's Ahelbarra said.

"They tell people that such agencies teach the girls of Pakistan vulgar and immoral ways to imbue them with a sense of rebellion against what has been, for many centuries, a very conservative country."