The Shaoguan city government said in a post on its microblog that the leak occurred at 3 a.m. at the Shaoguan Iron and Steel company's Songshan plant.
It said the rescue and recovery effort was underway and the cause of the leak was under investigation.
China has made considerable progress in improving industrial safety, but scores are still killed annually in the country's factories, coal mines and transportation networks.
In 2015, an explosion traced to improperly stored chemicals killed at least 173 people in the port city of Tianjin, about an hour east of Beijing.