"Security forces launched an extensive search operation in the mountains of Machh and Kolpur, killing eight militants," Deputy Commissioner of Bolan district, Abdul Waheed Shah said.
The search operation was launched after militants attacked the Rawalpindi-bound Jaffer Express train in Bolan district on Friday, killing four passengers and injuring 27 others including women and children.
The attack was followed by fierce clashes between security forces and armed militants.
"There was heavy exchange of fire as the militants resisted the search operation by the security forces high up in the mountains. The exchange continued for atleast five hours," Shah said.
More troops were called in the area to control the situation developed after the armed clash.
A senior government official said that helicopters were also used in the search operation which was still continuing.
"Two helicopters are hovering over mountains of Machh and other areas of Bolan," he said.
Train service between Balochistan and other parts of the country was temporarily suspended.
However Jaffar express was allowed to proceed toward Rawalpindi after alteration of engine.
Meanwhile, Bolan Mail from Karachi to Quetta was stopped at Notal area of Naseerabad for security reasons.
"We have been waiting for last five hours for departure," an awaiting passenger said.
The attack came just over a week after Baloch insurgents disguised as security personnel stopped buses at a fake check point near Machh and killed 13 civilians.
Balochistan, Pakistan's largest province by area, is wracked by violence by Baloch nationalist groups that want greater control over natural resources.
The banned Baluchistan Liberation Army claimed responsibility for the attack.