Gunmen armed with assault rifles and hand grenades raided the office of a Christian aid group in northern Pakistan Monday, killing six staffers, police said.

Five others were injured in attack on the World Vision office in the Ogay area of Mansehra district, 65 km northeast of Islamabad, district police chief Waqar Uddin said. All the victims were Pakistanis.

Rafaqat Ali, a junior police officer, said around 15 masked men first lobbed hand grenades into the building housing the World Vision office and then sprayed gunfire inside.

Ali said police was pursuing the assailants who fled to nearby mountains after the attack.

World Vision spokesman Rienk Van Velzen said the humanitarian organisation was suspending its activities in Mansehra district 'for the moment'.

'We are deeply sorry that we lost our six colleagues - two female and four male - in the incident,' Velzen said. 'We don't know why our office was attacked, our operations in that area are so much community based.

'We hope that we continue to work with the people of Pakistan to help them in building their own future.'

World Vision is among several international aid groups working in Mansehra since October 2005, when the region was hit by an earthquake that killed 79,000 people.

No one accepted immediate responsibility for the attack. Taliban and Al Qaeda-linked Islamist militants have targeted people working for foreign aid groups and US agencies in Pakistan, forcing many organisations to scale back operations.

Hundreds of Taliban militants are believed to have taken shelter in Mansehra after the army launched a successful offensive last year to dislodge Islamist insurgents.