21 passengers killed on Pakistani train after hundreds taken hostage

Twenty-one passengers were killed after a train in Pakistan was attacked and hundreds were taken hostage by the militant Balochistan Liberation Army on Tuesday, according to the Pakistani military.

Four military members conducting a rescue operation and 33 terrorists, members of the BLA, have been killed, according to the military.

The military operation has ended, according to a Pakistani military official.

On Tuesday, a U.S. official told ABC News at least 450 people were taken hostage on the train and said six Pakistani military personnel were killed.

The separatist militant group claimed it had taken 182 military and security personnel hostage on the train, according to a post on Telegram, but said they had released the majority of the civilians on board. The group claimed a higher number of casualties in the attack, saying they killed 20 Pakistani military personnel and shot down a drone.

The BLA had threatened to kill all the hostages if Pakistan's military tries to rescue them, the official said.

The BLA blew up part of the track, forcing the train to stop, before they boarded and took control, according to the official.

The attack happened in mountainous area right before a tunnel, making a rescue very difficult, they said.

Pakistani Interior Minister Mohsin Naqvi condemned the attack and said the government would not make any concessions to "beasts who fire on innocent passengers."

The train was trapped in a tunnel after the tracks were blown up and militants opened fire on it, reportedly injuring the driver, local authorities and police have told media.

The BLA believes the Balochistan region of Pakistan, in the country's far west bordering Iran and Afghanistan, should be allowed to break off from Pakistan. They are well-known for committing terror attacks in Pakistan. The BLA also attacked Jaffar Express trains in August 2013 and October 2016.

This is a developing story. Please check back for updates.