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 insurgents took over a passenger train carrying an estimated 500 people on Tuesday, with the separatists threatening to kill some 182 hostages if their demands are not met.