Pakistan's army says it has freed more than 300 hostages from a passenger train seized by militants in Balochistan province on Tuesday.

The military spokesperson said 33 militants were killed during the operation.

Twenty-one civilian hostages and four military personnel were killed by the Baloch Liberation Army (BLA) before the operation began, the military spokesperson said. These numbers have not been verified by the BBC, and it is unclear how many passengers are unaccounted for.

The military continues its search operation in the area to rule out any remaining threats.

There were approximately 440 passengers on board the train when it was attacked, according to the army's spokesperson.

Security officials have been quoted as saying some of the militants may have left the train, taking an unknown number of passengers with them into the surrounding mountainous area.

The military is working to find the passengers who escaped and fled into the surrounding area during the attack, the spokesperson said.

The Pakistani authorities - as well as several Western countries, including the UK and US - have designated the BLA as a terrorist organisation.

The BLA is one of the rebel groups demanding either greater autonomy or independence for Balochistan, Pakistan's largest province.

They accuse Islamabad of exploiting the province's rich mineral resources while also neglecting it. In the past, they have attacked military camps, railway stations and trains - but this is the first time they have hijacked a train.

At least 100 of those on the train were members of the security forces, officials have said.

The militants had threatened to kill hostages if authorities did not release Baloch political prisoners within 48 hours, according to local reports.

During the attack, the militants blew up a section of the tracks and opened fire on the train near a mountain tunnel.

Eyewitnesses described the "doomsday scenes" on board the train as the attack unfolded, with passenger Ishaq Noor telling the BBC: "We held our breath throughout the firing, not knowing what would happen next."

Officials had difficulty communicating with passengers at the time of the attack, because the remote area has no internet or mobile coverage.

Some passengers who managed to disembark from the train late on Tuesday evening walked for nearly four hours to reach the next railway station.

Among them was Muhammad Ashraf, who had been travelling from Quetta to Lahore to visit his family.

"We reached the station with great difficulty, because we were tired and there were children and women with us," he told the BBC.

Helicopters and hundreds of troops were deployed to rescue the hostages. More than 100 passengers had been freed by Wednesday morning.

The hijacking lasted more than 30 hours. Information relating to the attack and subsequent rescue operation has been tightly controlled throughout.

A spokesperson for the military said anyone involved in the attack would be brought to justice.

Additional reporting by Azadeh Moshiri.