Fire rises at a petrochemical factory after blasts caused by leakage of liquid hydrocarbon from tanks in Rizhao city, East China's Shandong province, July 16, 2015. [Photo/IC]

Zhang Zhenhua, a worker at the Rizhao Steel Holding Group, said the explosion was so strong that it smashed the windows in her office.

"After hearing a loud bang, we saw thick smoke rising. We had to leave our office immediately," she said.

The city's environmental protection bureau said that air quality monitors at the plant had found an excessive level of harmful non-methane hydrocarbon in some areas.

However, the density of the pollutant was dropping, the bureau added.