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.

Militants blew up a railway track and opened fire on the train on Tuesday as it travelled from Quetta, Balochistan’s capital city, to Peshawar in Khyber Pakhtunkhwa province.

The Baloch Liberation Army (BLA), an ethnic armed group, claimed responsibility for the attack and threatened to start executing hostages unless Baloch political prisoners, activists, and missing persons it said had been abducted by the military were not released within 48 hours.

The militants have made bombers wearing “suicide jackets” sit next to some of the hostages, said multiple sources who asked not to be named. They did not specify the number of people being held but the interior ministry said in a statement that an operation was underway to rescue them.

BLA said on Tuesday it was holding 214 people hostage, and a security source told Reuters that there were 425 passengers on the train when it was attacked.

The number of militants involved in the attack was not clear. The security sources said on Wednesday that 27 had been killed so far.

BLA is the largest of several ethnic armed groups battling Pakistan’s government in the mineral rich province of Balochistan, bordering Afghanistan and Iran.