The partial collapse of a coal mine in northern China has left nine people dead, state media reported Wednesday, reflecting the stubborn persistence of safety problems despite years-long efforts to reduce deaths in the sprawling sector.

The people killed were doing maintenance work inside the mine shaft when it collapsed Tuesday morning, the state-run Xinhua News Agency said. Rescuers were able to save one person, who is now in stable condition at a hospital.

The cause of the accident on the outskirts of Shuozhou city in coal-rich Shanxi province was under investigation. Local government officials did not immediately return phone calls from The Associated Press.

China is the world's largest producer and consumer of coal. Its mining industry has long been one of the world's deadliest, even as the government has strived to improve safety inside the mines and phase out older operations with outdated safety standards. A heavy reliance on coal is also blamed for the serious air pollution and smog that routinely smother northern cities in winter.

China's work safety administration reported 931 coal mine accident deaths in 2014, the last year for which figures were available, although official statistics are often questioned for their accuracy. The head of the agency has acknowledged that some mines cut corners on safety standards under financial pressure, while operators sometimes cover-up deadly accidents to avoid prosecution.

Government regulators have said they will shutter as many as 1,000 outdated and underperforming mines in coming years, even as China also tries to keep energy prices in check and prevent economic devastation in regions dominated by mining.