Three more workers remained trapped in the private Yongsheng coal mine in Heilongjiang province, while rescuers managed to pull out three other workers, the Xinhua News Agency reported.
Those killed included some workers who had entered the mine to pump out water, only to be overcome by the poisonous gas, the report said. Rescues managed to save 12 more of the workers, and were trying to reach the three rapped ones.
The incident was being investigated.
Last Thursday, authorities in southwest Guizhou province confirmed the deaths of all 13 people trapped in a coal mine for seven days following a blast. The cause of the blast was yet to be determined.
There were more than 60 such mine disasters in 2012, killing at least 245 miners. In one of the worst disasters last year, a mine explosion in August in southwest Sichuan province killed at least 40 miners.
Coal mine accidents in China take a heavy toll every year despite stringent safety regulations. Authorities in the past have blamed the deaths mostly on illegal mines or poor implementations of safety rules. Many of the illegal mines have already been shut down.