Dozens of people remained missing on Friday following a collapse at a coal mine in northern China that killed six, state media reported.

A rescue operation involving hundreds of workers was ongoing after a 180-metre-high slope gave way at the open-pit mine in the Inner Mongolia region's Alxa Left Banner area on Wednesday.

Emergency efforts were initially hampered after another landslide later that day.

According to state broadcaster CCTV, 47 people were still unaccounted for, six were recovered alive, while another six were found dead.

China's ministry of emergency management urged "all-out efforts to search for the missing personnel without delay, and not to lose hope of finding them", state news agency Xinhua reported Friday.

"Saving lives is still the top priority," Xinhua quoted a ministry work team as saying, adding that "efforts should also be made to prevent secondary disasters".

Footage from CCTV showed rescue workers in orange overalls and yellow helmets dwarfed by a mountain of rust-coloured rubble, and excavators working to clear some of the debris.

"I had just started work when I saw slag falling down the slope. The situation got worse and worse," a rescued worker named Ma Jianping told CCTV.

"We tried to organise an evacuation, but it was too late -- the slope came down," he said from a hospital bed in the neighbouring Ningxia region, a catheter protruding from his throat.

State media reported the collapse had affected a "wide area" of the mine operated by the Xinjing Coal Mining Company. It was not clear what caused the collapse.

CCTV said police were investigating, while Xinhua on Friday said the ministry of emergency management had called for "comprehensive investigations".

- 'I'd have died' -


Video posted on social media by a coal truck driver on Wednesday showed rocks cascading down a slope, kicking up clouds of dust that engulfed several vehicles.

"The whole slope has collapsed... How many people must be dead from that?" a male voice can be heard saying in the background.

"If I'd lined up over there today, I'd have died in there, too."

Located in China's arid north, Alxa League -- which includes the Alxa Left Banner -- is a sparsely populated region whose economy runs largely on mining and other extractive industries.

Mine safety in China has improved in recent decades, as has media coverage of major incidents, many of which were once overlooked.

Accidents still occur frequently, however, in an industry where safety protocols are often lax, especially at the most rudimentary sites.

Around 40 people were working underground when a gold mine in the northwestern Xinjiang region collapsed in December.

In 2021, 20 miners were rescued from a flooded coal mine in northern Shanxi province, while two others died.