The blast marked China's worst mining disaster in 17 years and authorities said the company involved had committed "serious" violations, with toxic gases accumulating in the mine.

The state news agency Xinhua said early Sunday an all-out rescue operation was under way to save the two people who were missing.

A total of 247 workers were underground at the time of the blast, which occurred at 7:29 pm (1129 GMT) on Friday at the Liushenyu coal mine in Shanxi province, according to Xinhua.

Of those, 128 people were sent to hospital for treatment, CCTV said.

Toxic and harmful gases under the mine shaft had exceeded safe limits for a long time, posing a risk of secondary disasters, officials told a news conference Saturday evening, according to Xinhua.

Preliminary findings showed the mining company had committed "serious illegal violations", authorities said in the press conference broadcast on CCTV.

A total of 755 emergency and medical personnel were dispatched to the site, CCTV said.

On Saturday evening rescue teams took turns going down into the mine shaft to look for the missing workers.

"As long as there is hope, we will make every possible effort," one rescuer told Xinhua.

Friday's explosion was the deadliest mining disaster in China since 2009, when 108 people were killed in a mine blast in northeast Heilongjiang province.

- Sulphur smell -

Injured miner Wang Yong told CCTV he heard no sound but smelled sulphur when the explosion happened.

"I didn't hear any sound at all, but then a cloud of smoke appeared," Wang said.

"When I smelled it, it was the smell of sulfur like when people set off firecrackers. When the smoke came down, I shouted for people to run," he said.

He recalled seeing people choked by smoke before he fainted.

"After more than an hour, I came to on my own, and then I woke up the person next to me" and got out, he told CCTV.

Helmeted rescuers were carrying stretchers at the site, with ambulances visible in the background, CCTV footage showed.

Hospitalised people lay in beds with bandages around their heads, the images showed. Others were on oxygen support.

Doctors provided oxygen, dehydration to reduce intracranial pressure and psychological treatment to patients, a nurse told CCTV.

President Xi Jinping urged "all-out efforts" to treat the injured and called for thorough investigations into the incident, according to Xinhua.

China's government vowed to severely punish those responsible and ordered a nationwide crackdown on illegal mining activities, Xinhua said.

"The State Council's accident investigation team will conduct a rigorous and uncompromising investigation," Xinhua said.

"Those found responsible will be severely punished in accordance with laws and regulations."

"All regions and relevant authorities are required to... launch tough crackdowns on illegal and unlawful activities", including the falsification of safety data, unclear headcounts of underground workers and illegal contracting, it added.

A person "responsible for" the company involved in the explosion has been "placed under control in accordance with the law", Xinhua said.

- Lax safety protocols -

Shanxi, one of China's poorer provinces, is the centre of the country's coal mining.

Mine safety in China has improved in recent decades, but accidents still occur in an industry where safety protocols are often lax and regulations vague.

In 2023, a collapse at an open-pit coal mine in the northern Inner Mongolia region killed 53 people.

China is the world's top consumer of coal and the largest greenhouse gas emitter, despite installing renewable energy capacity at record speed.