A massive explosion at an eastern China chemical plant with a long record of safety violations has killed at least 47 people and injured hundreds, 90 of them seriously.

Thursday's blast at the Tianjiayi Chemical plant in Yancheng is one of China's worst industrial accidents in years. Nearly 1,000 residents have been moved to safety as of Friday as a precaution against leaks and additional explosions, the city government said in a statement posted to its official microblog.

State-run television showed crushed cars, blown-out windows and workers leaving the factory with bloodied heads. Windows in buildings as far as about 6 kilometres were blown out by the force of the blast.

The city government statement said 3,500 medical workers at 16 hospitals had been mobilized to treat the injured, dozens of whom remain in critical condition. The blast created a crater and more than 900 firefighters were deployed to extinguish the fire that burned into the night.

The cause of the blast remained under investigation.

Chinese President Xi Jinping, on a state visit to Italy, demanded "all-out efforts" to find and rescue victims, the official Xinhua News Agency reported.

"Relief work must be well done to maintain social stability. Meanwhile, environmental monitoring and early warning should be strengthened to prevent environmental pollution as well as secondary disasters," it quoted Xi as saying.

Xi said local officials need to learn the lessons of a recent series of industrial accidents to preserve lives and property, signalling a likely crackdown on safety violations at a time when many Chinese companies are being hit by a downturn in sales that is squeezing profit margins.

The higher death toll, raised from 44 but with no change in the number of injured, suggested rescue crews were still finding bodies at the blast site.

China experiences frequent industrial accidents despite orders from the central government to improve safety at factories, power plants and mines.

Among the worst accidents was a massive 2015 explosion at a chemical warehouse in the port city of Tianjin that killed 173 people, most of them firefighters and police officers. The blast was blamed on illegal construction and unsafe storage of volatile materials.

In November, at least 22 people were killed and scores of vehicles destroyed in an explosion outside a chemical plant in the northeastern city of Zhangjiakou, which will host competitions in the 2022 Winter Olympics.