State news agency Xinhua said the blast happened in the air separation unit of the Henan Coal Gas Group factory, not in the gas tank areas, citing a source in the Yima city government. All production at the plant has been stopped.

“Many windows and doors within a 3km radius were shattered, and some interior doors were also blown out by the blast,” state broadcaster CCTV said on its Twitter-like Weibo social media account.

Videos obtained by local media showed a massive column of black smoke billowing from the factory and debris littering the road. Other images showed the doors and windows of homes blown out and shuttered shops with dented metal fronts.

A bloodied man was seen being helped out of a van in a video posted on social media.

The authenticity of the footage could not be immediately verified.

Deadly industrial accidents are common in China, where safety regulations are often poorly enforced.

The powerful explosion in the city of Yancheng toppled several buildings in the industrial park, blew out windows of nearby homes and even dented metal garage doors.

Authorities detained two dozen people in connection with the March 21 blast, which prompted the government to order a nationwide inspection of chemical firms.

Leaked chloroethylene came in contact with a fire source causing the explosion, authorities said in a February report, which also claimed the Chinese chemical firm responsible for the accident had concealed information and misled investigators.

Additional reporting by Zhenhua Lu