Pakistan’s military have been engaged in a deadly standoff for more than 24 hours with armed militants who hijacked a train and took hostages, in a dramatic escalation of an insurgency that has plagued the region for decades.

Pakistan forces launched a "full-scale" operation on Wednesday to rescue train passengers taken hostage by militants in the mountainous southwest, with security sources saying 155 had been freed in the past 24 hours.

Pakistani forces have rescued 155 passengers from a train that was hijacked by separatist fighters in the southwest of the country, security sources said on Wednesday, while the government said an operation was underway to rescue dozens still held hostage.