Nearly all personnel were stopped by police and checked for entry approvals before being allowed to proceed along the road leading to the mine.

A police officer said only those with government approval would be allowed access to the area. She said people living close to the mine had been sent to stay in a nearby town.

Security was also tight at another checkpoint in the neighboring region of Ningxia, 15 kilometers (9 miles) east of the mine, with dozens of officers inspecting every vehicle that sought to pass in either direction.

Cranes and other pieces of heavy equipment could be seen, along with covered trucks. It was unclear what the trucks were carrying.

Rescuers used heavy digging equipment and cameras that could snake down into the debris, along with thermal imagers and equipment to detect vital signs, Chang Zhigang, the head of the rescue operation, told reporters Thursday.

Crews were using extreme caution to avoid more secondary disasters, Chang said.

“We will continue to increase the rescue force, race against time, and do our best to search and rescue the lives of every trapped person,” he said.

Police are investigating the cause of the collapse and some people have been detained, Chang said.

“We will publicize the result of the investigation in due course,” he said.

The initial cave-in of one of the pit’s walls occurred at about 1 p.m. Wednesday, burying people and mining trucks below in tons of rocks and sand. It was followed about five hours later by the additional landslide, forcing the work suspension.

The official Xinhua News Agency said about 900 rescuers with heavy equipment were on the scene and they had resumed the search by Thursday morning.

Chinese President Xi Jinping called for “all-out efforts in search and rescue” and for “ensuring the safety of people’s lives and property and maintaining overall social stability.”

Images of the collapse distributed by CCTV showed a massive wall of debris rushing down a slope onto people and vehicles below.

The company running the mine, Inner Mongolia Xinjing Coal Industry Co. Ltd., was fined last year for multiple safety violations including insecure routes into and out of the pit, unsafe storage of volatile materials and a lack of training for its safety staff, according to the news website The Paper.

Inner Mongolia is a key region for mining of coal and various minerals and rare earths that critics say has ravaged the region’s landscape of mountains, grassy steppes and deserts.

China overwhelmingly relies on coal for power generation but has tried to reduce the number of deadly mine accidents through a greater emphasis on safety and the closure of smaller operations that lacked necessary equipment.

It has recorded a slew of deadly industrial and construction accidents in recent months as a result of poor safety training and regulation, official corruption and a tendency to cut corners by companies seeking to make profits.

Despite those high-profile incidents, the overall number of industrial accidents fell by 27% in 2022, when much of China’s economy was shut down under its “zero COVID” policy, from the previous year, the Ministry of Emergency Management announced last month. The number of deaths in the accidents also fell 23.6%, the ministry said.