Pakistani officials say a bomb has ripped through a passenger van in the country's northwest tribal belt, killing at least six people.

The blast Wednesday in the town of Jamrud in Khyber district left a mangled, charred shell of the van and a crater in the ground.

No group immediately claimed responsibility for the bombing.

The lawless tribal belt near the Afghan border is a stronghold of Taliban- and al-Qaida-linked militants.

The United Nations refugee agency said last week more than 100,000 people have poured into a camp it runs near Peshawar this year after fleeing the tribal region, with most coming from Khyber district.

Families were fearful of being caught up in a Pakistani military campaign to rout insurgents in the area, and a UNHCR spokesman said people told staff they had been advised by military officials to leave.