A bomb attack killed at least seven members of a Pakistani pro-government militia and wounded 13 others on Thursday as they were travelling to a garrison town in the northwest, officials said.

Twenty militiamen were travelling by bus from the tribal district of Khyber, where fighting has recently intensified in a long-running Pak military operation against the Taliban and other insurgents on the Afghan border.

A roadside bomb struck the bus in the village of Hassanzo in the neighbouring tribal district of Orakzai as it was on its way to Kohat, one of the main garrison towns for the Pakistan army in the northwest, officials said.

"As a result of the blast, the passenger van was completely destroyed and seven people were killed and 13 others are wounded," top Orakzai administration official Mehmood Hasan told AFP, warning the death toll could rise.

"According to initial information, the van was hired by a local peace committee from Khyber and 20 members of the peace committee were on board together with the driver," he said.

Peace committees are armed groups operating in the restive northwest with the aim of protecting local communities on behalf of the government.

Security officials confirmed the attack and the death toll.

Pakistani troops have been locked in deadly battles for years with domestic insurgents in the northwestern tribal belt, where Taliban and Al-Qaeda-linked militants also plot attacks on Afghan and Western targets.

Islamabad says more than 35,000 people have been killed as a result of terrorism in the country since the 9/11 attacks on the United States.