The miners began the protest on Monday at the Djurdjevik mine, between 100m and 250m (330-820ft) below ground.
The BBC reports that the miners are demanding wage increases and are angry at the recent hiring of an administrative official.
Union leader Said Muhic said they were refusing to negotiate with the managers and threatening to go on hunger strike.
Muhic told state radio that the underground strike was triggered by the miner’s board appointing a company secretary.
He said the move violated an agreement with the government to hold off recruitment of administrative staff until the miners' wages went up.
"The situation is really dramatic," he said.
The Djurdjevik mine employs more than 1000 people and produces about 600,000 tons of coal per year for a local power plant.
The protest follows a similar action by more than 1000 Romanian miners last month who were protesting against a state-holding company's refusal to honor price agreements.