Pakistani police have reported a militant incident at the offices of a US-based Christian aid group.

Hand grenades were thrown into the offices of a World Vision office in Mansehra in Pakistan's North West Frontier Province, killing five people.

Those who died were Pakistani employees at the office, who were targeted by about 15 militants who stormed the building.

After being alerted, police exchanged fire with the attackers, who later fled the scene.

The Mansehra World Vision office had been providing aid to survivors of an earthquake that killed 75,000 people in Pakistan in October 2005.