Drone footage from the country's emergency management ministry shows an industrial area outside Yancheng city in Jiangsu province covered in firefighting foam, with storage tanks turned to twisted metals and charred factory buildings with roofs torn open.
In another clip, massive flames and thick smoke come out of the factory and rise into the air.
Executives of the chemical plant have been taken into police custody.
"He extends his honest condolences to the families of the victims and wishes those injured a speedy recovery", Farhan Haq, deputy spokesman for the secretary-general, said in a statement.
Authorities must step up action to prevent such incidents from happening and find out the cause of the blast as quickly as possible, Mr. Xi added.
The biggest accident in recent years was the August 2015 Tianjin explosion, which killed more than 160 people and injured almost 1,000. Wang Xinfang said shards of glass from the windows had been "falling like rain" in a village six kms away from the plant.
Smoke billows from fire behind a damaged building following the explosion
Thursday's blast at the Tianjiayi chemical plant in the city of Yancheng is one of China's worst industrial accidents in years.
Previously, the local fire department said in a post on its official Weibo account that at least 12 people who were injured in the explosion on Thursday have been rescued by firefighters, while 6 died. Meanwhile the "Beijing Youth Daily" reported that the explosion was caused by benzene and occurred in a production area of the plant.
The company, Tianjiayi Chemical Co - which produces more than 30 organic chemical compounds, some highly flammable - had a history of safety violations and had been punished repeatedly, state news agency Xinhua said.
The fire was brought under control at around 0300 local time on Friday, state TV said. Another clip showed rescuers walking around a darkened neighbourhood as a voice says "the team is here to look for anyone who might be trapped".
It was still unclear what caused the pesticide plant to explode, but the company behind the operation had been cited and fined in the past for violating several work safety protocols.
An inspection of chemical factories in the Yangcheng area after a 2017 explosion at a different facility killed 10 people discovered over 200 safety hazards, ranging from leaky pipes to inadequately-trained employees.
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