Three miners were rescued as the work was still underway to save other three person trapped inside the mine in Heilongjiang province.
Those killed included three miners who entered the Yongsheng colliery to pump out water last night, but poisonous gas caused them to faint, state-run Xinhua reported.
Rescuers were sent to the site, but some of them were also affected by the toxic gas.
Rescuers said the poisonous gas may have been carbon monoxide.
