The CEO of a South Korean lithium battery manufacturer apologised on Tuesday following a massive factory fire that killed 23 workers, but said the company had complied with all required safety precautions and training, Paralel.Az reports citing Reuters.

The fire on Monday, which began at a factory with 35,000 lithium batteries, produced thick smoke that spread quickly and the workers inside the second-floor location likely lost consciousness and succumbed within seconds, fire officials said.

Firefighters with search dogs combed the gutted structure on Tuesday in Hwaseong, an industrial cluster southwest of the capital Seoul, and found the last person who had been unaccounted for, raising the death toll to 23.

Seventeen of those who died were Chinese, and one Laotian was among those killed. Most of the workers were hired temporarily at the plant packing primary lithium batteries run by South Korea-based Aricell, which is majority-owned by S-Connect.

Aricell CEO Park Soon-kwan offered condolences to the workers who were killed and apologised to everyone who had been affected by the accident.

"We will be conscientiously taking part in the investigation by authorities and will do our best to determine the cause of the accident and to take measures to prevent a repeat of such an accident," Park told reporters at the scene of the fire.

Officials from agencies including the National Forensic Service, police and the fire department entered the factory as part of a joint investigation.

The fire was the latest industrial accident in a country where dozens of manufacturing workers lose their lives on the job each year despite repeated calls to improve workplace safety.

"I ask the ministries of labour and industry and the National Fire Agency to conduct an urgent safety inspection and, where there is concern of an accident, take immediate measures," Prime Minister Han Duck-soo said at a cabinet meeting.

Park, the Aricell CEO, said the company had fully complied with safety procedures and training, but some of the 103 workers at the factory including some of those killed were contract workers dispatched by a manpower company.