The officials said the victims were all unarmed civilians working in coal mines.
At least two more persons were killed, as security forces fired back at attackers, according to the union members, but the police maintained that one person died in the incident.
Leader of the House Piyush Goyal also said the matter is serious. "The government is seized of the matter. The government will make a detailed statement on this issue," he said.
At least 14 civilians and one soldier were killed in firing by security forces in a botched anti-insurgency operation and retaliatory violence within 24 hours in Mon district of the state, as per police.