There were no reports of any casualties in the mudslide.
Coal is a major source of energy in China but its mines are among the world’s deadliest, largely due to lax enforcement of safety standards, despite repeated government orders for improvements in safety over the years.
Inner Mongolia is a major coal-producing region. China’s mines have been trying to boost output over the past year under a government call for greater supplies and stable prices.
Before the mudslide, four rescue teams of 109 people were searching for the trapped miners, state media reported.
Authorities also sent 238 firefighters, 41 fire trucks and six rescue dogs to join the rescue and more teams of about 200 people were expected to arrive on Thursday, state media said.
Premier Li Keqiang demanded a quick investigation into the cause of the collapse.
The accident was the top trending discussion on the Weibo, social media platform on Thursday with some users saying most of the missing were dump truck and excavator drivers.
The National Health Commission said on Wednesday evening six injured people had been rescued and it had sent 15 ambulances and 45 medical staff to help with the rescue.