Pakistani security forces are carrying out a rescue operation to free dozens of passengers taken hostage by separatist militants who hijacked their train in the southwest of the country on Tuesday.

Here is a look at what happened, and the current situation.

THE TRAIN
The Jaffar Express was heading to the city of Peshawar in Khyber Pakhtunkhwa province from Quetta, the capital of Balochistan province, with 425 people on board when it was attacked.

The passengers included personnel from the Pakistani army and other security forces who were traveling on leave.

THE ATTACK
Militants blew up the railway tracks and opened fire on the train, killing the driver and trapping the locomotive inside a tunnel before taking control of it.

Some of those rescued said they crouched low when the firing started, while others spoke of passengers who had been injured or killed.

RESCUE OPERATION
Security forces have launched a massive operation to free the hostages, deploying special forces and helicopters, and 155 passengers have been rescued so far.

Some 27 militants have also been killed, sources say.

However, attackers wearing suicide bombs, who have been seated next to some of the hostages, have made the rescue efforts tougher.

THE DEMANDS
Ethnic armed group Baloch Liberation Army (BLA), which has claimed responsibility for the attack, has demanded the release of Baloch political prisoners, activists, and missing persons who it says were abducted by the military, within 48 hours.

It has threatened to start executing the hostages if the government does not fulfil the demand.

GOVERNMENT RESPONSE
Pakistan’s prime minister, Shehbaz Sharif, has condemned the attack and said security officials are “repelling” the militants.

Interior Minister Mohsin Naqvi has called the attackers “enemies” of Pakistan and vowed to foil their conspiracy to destabilize the nation.