SIBI, Pakistan - 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.

More than 450 passengers were on board when militants captured the train at the entrance of a tunnel in a remote frontier district, with an unknown number of hostages still being held. 

"Information suggests that some militants have fled, taking an unknown number of hostages into the local mountainous areas," a security official in the area told AFP.

Militants bombed a section of the railway track and stormed the train on Tuesday afternoon in southwest Balochistan province, which borders Iran and Afghanistan, where attacks by separatists have risen sharply in the past year.

According to security sources, the "terrorists have positioned suicide bombers right next to innocent hostage passengers".

Three people have been killed, including the train driver, during the siege in mountainous Sibi district.

A security official told AFP "a full-scale operation" would aim to free the rest of the captives.

"Security forces have safely rescued 155 passengers... 27 terrorists have been eliminated," a security source said.

Muhammad Kashif, a senior railway government official in the provincial capital Quetta, told AFP on Tuesday afternoon that the 450 passengers on board had been taken hostage. 

It was not immediately clear how many people remained on board, but passengers who spent hours walking through rugged mountains to reach safety described being set free by the militants.

"Our women pleaded with them, and they spared us," Babar Masih, a 38-year-old Christian labourer told AFP on Wednesday. "They told us to get out and not look back. As we ran, I noticed many others running alongside us."

At a railway station in Quetta, paramilitary troops brought empty coffins that will be sent to the site of the incident.

"I can't find the words to describe how we managed to escape. It was terrifying," Muhammad Bilal, who had been travelling with his mother on the Jaffar Express train, told AFP.

Outsiders identified 

The assault was immediately claimed by the Baloch Liberation Army (BLA), a separatist group that has staged a series of recent attacks against security forces and ethnic groups.

The group has demanded an exchange with security forces for its imprisoned members. 

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