There are concerns that aid groups are becoming targets for militant groups in Pakistan, after six aid workers were killed when militants struck a World Vision office in the North-West Frontier Province.

It is not the first time an aid group has been targeted and World Vision has suspended its operations in Pakistan. It is likely other groups will soon follow.

World Vision said the office was targeted because it was running programs to help women.

A group of about 15 gunmen stormed into the offices in the Mansehra district on Wednesday.

Witnesses say they gathered the staff together and then dragged them off one by one to a separate room, where they were executed.

Six Pakistani aid workers, including two women, were killed, and about six others were badly injured.

World Vision Australia chief executive Reverend Tim Costello says the group has now suspended its operations in Pakistan indefinitely.

"In the past there have been threats and the community have said to us, 'Let us deal with it, we want you here'," he said.

"So it's not as if there isn't a very strong basis of support within the community to do what we're doing.

"But in this instance no-one, it appears, could protect our staff and that's why then you have to say, whatever assurances, whatever the urgency and the priority and the need, you have to guarantee [the safety of] those staff."

It is not the first time an aid organisation has been targeted in the area. In February 2008, four local staff with the British-run aid group PLAN International were killed in a similar attack in the same area.

'No safety guarantee'

World Vision has been operating in the region since a massive earthquake struck the area in October 2005.

Reverend Costello says it is clear that militants are targeting aid workers.

"Employing women is an important statement, giving them opportunities for businesses, for education ... but it's provocative," he said.

"Most aid organisations have exactly the same philosophy and therefore are exposed to the same vulnerabilities."

Attacks by militant groups have killed more than 3,000 people in Pakistan since 2007.

In recent months, there has been a spike in the violence as the Pakistani military has conducted an offensive against the Taliban along the border with Afghanistan.

Reverend Costello says this recent attack will make all aid organisations nervous about operating in Pakistan.

"The tragedy is in the last 10 to 15 years, aid workers have become targets, and that wasn't the case before," he said.

"So the assessment really is now about a community that's desperately poor, that wants us there, but if you can't guarantee the safety of your staff you can't stay."