Multiple powerful explosions set a lithium battery factory on fire in South Korea on Monday, killing 22 workers, most of them Chinese nationals, as it burned out of control for nearly six hours, fire officials said.

The blaze ripped through a factory run by primary battery manufacturer Aricell in Hwaseong, an industrial cluster southwest of the capital Seoul. It was eventually largely extinguished.

Eighteen Chinese workers and one Laotian were among the dead. The nationality of the remaining deceased worker was not yet confirmed, fire official Kim Jin-young told reporters, citing information from company officials.

The blaze began at 10:31 a.m. (0131 GMT) after a series of battery cells exploded inside a warehouse with some 35,000 units, Kim said. What had triggered the explosion remains unclear, he said.

A Reuters journalist saw firefighters moving up to six bodies out of the factory. Due to the intensity of the blaze, rescuers were finding it difficult to identify the dead, Kim said.

Two people were being treated for major burns, officials at the scene said.

Live TV footage showed firefighters spraying the damaged steel and concrete building. Parts of the upper level had collapsed, and large chunks of the building looked like they had been blown out into the street by explosions.

Aerial footage showed massive white smoke clouds billowing from the structure and explosions rolling through the building.

Kim Jae-ho, Fire and Disaster Prevention professor at Daejeon University, said the fire had probably spread too quickly for workers to escape.

“Battery materials such as nickel are easily flammable,” he said. “So often, there is not enough time to respond, compared to a fire caused by other materials.”

President Yoon Suk Yeol visited the scene of the accident later on Monday. Interior Minister Lee Sang-min called on the local authorities to take steps to prevent any hazardous chemicals from contaminating the surrounding area.

Established in 2020, Aricell makes lithium primary batteries for sensors and radio communication devices. It has 48 employees, according to its latest regulatory filing and its Linkedin profile.

Calls to Aricell offices were unanswered.

The company is not listed on South Korea’s stock market but is majority owned by S-Connect, according to the Aricell regulatory filing. S-Connect is registered on the junior Kosdaq index and its shares closed down 22.5%.

Park Chul-wan at Seojeong University said Aricell is known to use extremely toxic material, which could explain the high death toll.

“The fact that there were so many casualties when this was on only the second floor is because of the toxic materials and not so much because of burns,” he said.

South Korea has made efforts to improve its safety record after several past accidents, many of which have been blamed on negligence.

Each year dozens of workers are killed in industrial accidents, which prompted parliament to pass a law aimed at punishing the executives of a company in the event of a fatal accident with possible jail terms.

South Korea is also home to major producers of lithium-ion batteries that power electric vehicles (EVs) and the world’s fifth largest automakers, Hyundai Motor Group, which are making a push to shift away from internal combustion cars to EVs.