A magnitude 2.2 earthquake was believed to have been caused by the blast (Photo: Xinhua)

Thursday's blast at the Tianjiayi Chemical plant in the city of Yancheng was one of China's worst industrial accidents in years. Nearly 1,000 area residents had been moved to safety as of Friday as a precaution against leaks and additional explosions, the city government said in a statement. 3,500 medical workers at 16 hospitals have been mobilized to treat the injured, dozens of whom are in critical condition.The cause of the blast is under investigation.