The death toll from a militant hijacking of a train in Pakistan's southwestern mountains has risen to 31 soldiers, staff and civilians, the military said on Friday as it accused India and Afghanistan of backing the insurgents.

Terrorists in explosive vests have reportedly threatened Pakistan train passengers' lives if officials refuse to release imprisoned militants.

Pakistani officials say insurgents who attacked a passenger train carrying 440 people in the southwest killed 21 hostages but security forces rescued the others.