The collapse on Wednesday at an open-pit mine in the Alxa League operated by Xinjing Coal Mining Co left a pile of debris about 500 metres across and an estimated 80m high, state media reported.
"I had just started work at 1.15 in the afternoon when I realised that rocks were falling from the mountain," a miner told state broadcaster CCTV from hospital on Thursday.
"I saw that the situation was getting more and more serious, and an evacuation was organised, but it was too late, the mountain just collapsed."
President Xi Jinping on Wednesday ordered search and rescue efforts, state media reported, although a second landslide in the evening hampered the work to find survivors.
"We must make every possible effort to rescue the missing persons and treat the injured," Xi said.
Premier Li Keqiang also demanded a quick investigation into the cause of the collapse.
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.
China's mines have also been trying to boost output over the past year under a government call for greater supplies and stable prices. Inner Mongolia is the country's top coal-producing region.
Three hundred fire rescue personnel, 60 fire engines and 6 search and rescue dogs were at the scene on Thursday to aid the search for trapped miners, state media said.
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.
The previously underground mine was converted into an open-pit operation in 2012, according to state media. It had suspended production for three years before restarting in April 2021, state media said.
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