Pakistani troops freed dozens of train passengers taken hostage by Baloch Liberation Army (BLA) armed militants in the country's southwest on Tuesday.

However, hundreds more are still being held in the deadly siege.

Heavy gunfire ensued between security forces and the militants.

"Security forces have successfully freed 80 hostages, including 43 men, 26 women, and 11 children, from the terrorists," security sources told AFP, adding that 13 militants had been killed.

"Efforts are ongoing to ensure the safe release of the remaining passengers. The terrorists have been surrounded, and the operation will continue until the last terrorist is neutralized."

A nearby railway station in Mach has been turned into a makeshift hospital to receive some of the wounded.

The driver of the train, a police officer, and a soldier were all killed in the assault, according to paramedic Nazim Farooq and railway official Muhammad Aslam, both at Mach railway station.

The Baloch Liberation Army (BLA) which seeks independence for Balochistan from Pakistan in a statement claimed responsibility for taking control of a train and keeping over 180 passengers, most of them Pakistani soldiers, hostages.

Several Pakistani military personnel were also killed, the BLA said.

The Jaffar Express, with over 450 passengers on board in nine bogies, was on its way from Quetta in Pakistan's southwestern Balochistan province to Peshawar in Khyber Pakhtunkhwa when it was fired on, railway officials told news agency Reuters.

"Over 450 passengers onboard are being held hostage by gunmen," Muhammad Kashif, a senior railway government official in Quetta, told news agency AFP earlier.

In a statement signed by its spokesperson Jeeyand Baloch, the BLA said the hostages would be killed if Pakistani forces launched an operation.

"Any military incursion will be met with an equally forceful response. So far, six military personnel have been killed, and hundreds of passengers remain under BLA custody. The Baloch Liberation Army takes full responsibility for this operation," the BLA spokesperson said in the statement shared on social media.

The BLA fighters blew up the railway tracks and forced the train to stop inside a tunnel, after which they boarded it.

Security personnel were present on the train, and attackers exchanged gunfire after the attack.

Several ambulances have been rushed to the spot.