The separatist Baloch Liberation Army claimed responsibility for the attack Tuesday in a tunnel in Bolan, a district in restive Balochistan province.

Pakistan’s military have been engaged in a deadly standoff for more than 24 hours with armed militants who hijacked a train and took hostages, in a dramatic escalation of an insurgency that has plagued the region for decades.

The BLA is one of the largest militant outfits demanding independence for Balochistan. It is designated as a terrorist group by Pakistan as well as the US.