The blast occurred on Thursday at the Chenjiagang Industrial Park in the city of Yancheng, in Jiangsu province, and the fire was finally brought under control at 3am on Friday, state television said.

Survivors were taken to 16 hospitals with 640 people being treated for injuries. Thirty-two of them were in a critical condition, it said.

The fire at a plant owned by the Tianjiayi Chemical Company spread to neighbouring factories. Children at a kindergarten in the vicinity were also injured in the blast, media reported.

The cause of the explosion was under investigation, but the company - which produces more than 30 organic chemical compounds, some of which are highly flammable - has been cited and fined for work safety violations in the past, the China Daily said.

Police, some wearing face masks, sealed off roads to the plant. The power of the blast smashed windows in one village a few kilometres away.

"We only moved three years ago from the place where the explosion happened, so we'd have been blown up if we were still there," one resident, who gave his family name as Wang, told Reuters.

President Xi Jinping, who is in Italy on a state visit, ordered all-out efforts to care for the injured and to "earnestly maintain social stability", state television said.

Authorities must step up action to prevent such incidents from happening and find out the cause of the blast as quickly as possible, Xi added.

"There have recently been a series of major accidents, and all places and relevant departments must fully learn the lessons from these," the report cited Xi as saying.

Jiangsu will launch inspections on chemical producers and warehouses, according to an emergency notice published by official media on Friday.

The notice, published on the news website of Jiangsu province's Communist Party, said the government would shut down any chemical firms found not complying with regulations on dangerous chemicals.

Public anger over safety standards has grown in China over industrial accidents ranging from mining disasters to factory fires that have marred three decades of swift economic growth.

In 2015, 165 people were killed in a series of explosions at a chemical warehouse in the northern city of Tianjin.

The explosions at Tianjin, one of the world's busiest ports and not far from the capital, Beijing, were big enough to be seen by satellites and register on earthquake sensors.

Australian Associated Press