South Korea's fire department on Monday said 18 of the 22 killed in a massive blaze at a lithium battery factory were Chinese nationals.

More than 100 people were working in the factory when workers heard a series of explosions from the second floor, where lithium-ion batteries were being inspected and packaged, firefighter Kim Jin-young told media.

In the massive blaze that ensued, twenty-two people were killed, including 20 foreign nationals -- 18 Chinese, one from Laos, and one of unknown nationality, he said.

"Most of the bodies are badly burned so it will take some time to identify each one," he added.

Firefighters were still searching for one more person who remained unaccounted for, Kim said, adding they had managed to contain the largest blaze at the plant and get inside.

Firefighters were "doing cooling operations to prevent the fire from expanding to nearby factories," Kim said.

Images shared by Yonhap after the fire broke out showed huge plumes of billowing grey smoke rising into the sky above the factory, with orange flames inside the building.

The vast factory had an estimated 35,000 battery cells on the second floor in storage, with more batteries stored in other areas.

Lithium batteries burn hot and fast, and are difficult to control with conventional fire extinguishing methods.

"Due to fears of additional explosions, it was difficult to enter," Kim said, describing the tricky rescue operation.

"As it is a lithium battery manufacturer, we (had) determined that spraying water will not extinguish the fire, so we (used) dry sand," he added.

The lithium battery plant is owned by Aricell, a South Korean primary battery manufacturer. It is located in Hwaseong city, just south of the capital Seoul. (AFP)