State media said the State Council, China's Cabinet, had been ordered to oversee the investigation.

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. That 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.

Thursday's disaster occurred at a factory run by the Jiangsu Tianjiayi Chemical Co. Located among a cluster of chemical factories in Yancheng, it has a dismal safety record: In February 2018, China's State Administration for Work Safety cited 13 types of safety hazards at the company, including mishandling of tanks of toxic benzene, the source of Thursday's explosion.

Those violations came despite the plant having racked up 1,790,000 RMB ($267,000) in fines since 2016 for violations of environmental regulations, according to a judgments issued by local county and city environmental protection bureaus. Those included improperly dealing with hazardous waste and evading air pollution supervision.

A 2017 explosion that killed 10 at a nearby plant prompted China's national safety regulator, the State Administration of Work Safety, to dispatch inspectors. They discovered over 200 safety hazards at chemical factories in Yancheng and four nearby cities, including 13 at the Tianjiayi plant. Safety hazards cited included leaks and drips, employees who didn't understand safety procedures, and a lack of emergency shut-off valves on tanks carrying flammable chemicals.

In 2014, the company's chairman, Zhang Qinyue, and Wu Guozhong, its former supply chief, were arrested on suspicion of dumping and burying hazardous waste byproducts near a temple and a village landfill, according to a Jiangsu court criminal judgement. They were convicted in 2017 and the company was fined 1,000,000 RMB ($149,000).

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

Thursday's blast in an industrial park in the city of Yancheng, north of Shanghai, was one of China's worst industrial accidents in recent years. State-run television showed crushed cars, blown out windows and workers leaving the factory with bloodied heads.

Schools were closed and nearly 1,000 residents were moved to safety as a precaution against leaks and additional explosions, the city government said in a statement posted to its microblog.

The blast created a crater and more than 900 firefighters were deployed to extinguish the fire that burned into the night.

Windows in buildings as far as about 6 km away were blown out by the force of the blast, which caused a magnitude 2.2 seismic shock.

A resident of the community of Chenjiagang about 5 km from the plant, said neighbours were injured by glass from windows smashed by the blast force.

'At the time of the explosion, I was almost deafened and I was terribly frightened,' said the woman, who gave only her surname, Zhi.

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,' the report quoted Xi as saying.

Local officials need to learn the lessons of a recent series of industrial accidents to preserve lives and property, Xi said, signaling a likely crackdown on safety at a time when many Chinese firms 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.

The Yancheng city government statement said 3,500 medical workers at 16 hospitals were mobilised to treat the injured, dozens of whom remained in critical condition.

The cause of the blast was under investigation and people responsible for operations at the plant have been placed 'under control', Xinhua said. It wasn't clear whether anyone had been formally arrested.

In this aerial photo released by China's Xinhua News Agency, damaged buildings are seen at the site of the factory explosion on Friday. AP