Pakistani security forces clashed with hundreds of separatist militants on Wednesday in an attempt to rescue around 300 hostages from a train in the country’s remote southwest, according to officials.

Security forces were being cautious as officials said the hostages were surrounded by militants wearing vests loaded with explosives.

Government spokesman Shahid Rind said Pakistani, who were being backed up by helicopters in the remote region, were being cautious because of the situation. He described the attack “an act of terrorism.”

At least 27 militants have been killed and security forces rescued more than 150 of the 450 people who were on the train when it was hijacked on Tuesday as it entered a tunnel in Bolan, a district in restive Balochistan province.

It was the first time militants have ever carried out such an attack.

The Baloch Liberation Army group has claimed responsibility for the attack, saying it was open to negotiations to swap prisoners. So far, there has been no response or any indication from the government to the offer from the insurgents.

The BLA regularly targets Pakistani security forces, but has also in the past attacked civilians, including Chinese nationals working on multibillion-dollar projects related to the China-Pakistan Economic Corridor, or CPEC.

Pakistan hosts thousands of Chinese workers as part of Beijing’s multibillion-dollar Belt and Road Initiative, which is building major infrastructure projects.

Authorities said the rescued included women and children, while an undisclosed number of security personnel have been killed, according to three security officials who spoke on condition of anonymity as they were not authorized to speak to media.

According to the officials, the train was partially inside the tunnel when the militants blew up the tracks and immobilized the engine and its nine coaches.

The driver was critically wounded by gunfire and guards aboard the train were attack, although the officials gave no details about the number of guards that were aboard or if they had been killed.

Rescued passengers were being sent to their home towns and injured are being treated at hospitals at Mach district in Balochistan. Others were taken to Quetta about 62 miles away.

The train that was traveling from the provincial capital, Quetta, to the northern city of Peshawar when the attack took place.

Balochistan, which borders Iran and Afghanistan, has long been a hotspot for separatist insurgencies, with militants demanding greater autonomy and a larger share of the region’s natural resources.

The BLA, which has waged a yearslong insurgency, said the hostages and some captured members of the security forces were being guarded by suicide bombers are guarding them.

BLA has warned that the life of hostages would be at risk if the government does not negotiate.

BLA spokesman Jeeyand Baloch said in a statement on Tuesday night that the group was ready to free passengers if the government agrees to release the group’s jailed militants.

Since then, no government officials have been available for comment. Such demands have been rejected in the past.

Trains in Balochistan typically have security personnel on board as members of the military frequently use trains to travel from Quetta, the capital of Balochistan to other parts of the country.

Militants have attacked trains in the past but have never managed to hijack one.

In November, a separatist group carried out a suicide bombing at a train station in Quetta that killed 26 people.

Pakistani authorities and analysts estimate that the BLA has around 3,000 fighters.

Oil- and mineral-rich Balochistan is Pakistan’s largest and least populated province.

It’s a hub for the country’s ethnic Baloch minority, whose members say they face discrimination and exploitation by the central government.