A cooking gas explosion at a barbeque restaurant in northwest China has killed 31 people, and injured seven others.

China Central Television says the explosion occurred at around 8:40 p.m. on Wednesday in Yinchuan, the main city of the Ningxia Hui Autonomous Region.

Local authorities say a restaurant employee smelled a gas leak about an hour before the explosion, and discovered a broken valve on a gas tank. The blast reportedly occurred while the employee was replacing the valve.

Chinese media reports say the restaurant is popular with local people, and was crowded at the time of the explosion.

Rescue workers were quoted as saying that most of the victims died of suffocation. Several high school students and retirees were among the dead.

Chinese President Xi Jinping called for all-out efforts in treating the injured people. He urged officials to identify the cause of the accident as soon as possible, and to hold people accountable under the law. He also called for stronger safety oversight in key industries.

The local authorities say nine people, including the manager of the restaurant, have been detained as part of the investigation into the explosion.

The deadly accident occurred on the eve of a three-day national holiday, when many people are expected to go out.