The accident happened at around 2pm in Kampene, 180 kilometres south of town of Kindu, when "dozens of people, including children and pregnant women" were at work on the site, a civil society campaigner said.
Iron ore miner Vale SA and regulators under fire after a mine dam collapse in Brumadinho town killed dozens and left some 300 others unaccounted for.
A total of 87 people were working underground in the Shaanxi province mine at the time of the accident on Saturday afternoon.
The construction workers were trapped underground by smoke after a fire broke out in mine operated by fertilizer company Uralkali in Perm region, a day ago.
The explosion occurred due to the accumulation of methane gas inside the coal mine in Marwaarh, 45 kilometres east of Quetta, which is the capital of Balochistan province, officials said.
The tragedy worst ever mining disaster to take place since May 2014 when 301 miners were killed in Turkey's western town of Soma.
Police officials said they were testing the poisonous gas that killed the workers and had detained an unspecified number of people in connection with the incident.
About 500 people work at Zemestan-Yurt coal mine in northeastern Iran, and over 80 are believed to be trapped inside, local media reports.
Gas explosion in coal mine in northwest Pakistan has killed three miners and left several others trapped
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