Two miners were killed and four others were still missing in a coal mine gas blast accident happened on Sunday in North China's Inner Mongolia Autonomous Region.

The regional government source said the blast occurred at 11:30a.m. in Songshutan Colliery in Alashan League.

The rescue operation was slowed down due to the high gas density underground.

Two miners were already found dead but the whereabouts of four others are still unknown, said the government source.

Investigation into the cause of the accident has already started.