The fire began after batteries exploded while workers were examining and packaging them at the second floor of the factory in Hwaseong city, just south of Seoul, on Monday morning, fire officials said.
They said they would investigate the cause of the blaze.
Firefighters retrieved bodies from the burnt-out factory following the fire. (AP PHOTO)
The dead included 18 Chinese, two South Koreans and one Laotian, local fire official Kim Jin-young told a televised briefing.
He said the nationality of one of the dead could not be immediately verified.
In the past few decades, many people from China, including ethnic Koreans, have migrated to South Korea to seek jobs.
Like other foreign migrants from Southeast Asian nations, they often end up in factories or in physically demanding and low-paying jobs shunned by more affluent South Koreans.
Kim also one factory worker remained out of contact and rescuers continued to search the site.
He said that two of the eight injured were in serious conditions.
The fire started at one of the factory buildings owned by a company named Aricell.
Kim said the victims likely failed to escape via stairs to the ground. He said that authorities will investigate whether there were fire extinguishing systems at the site and if they worked.
Prime Minister Han Duck-soo, top centre, visited the factory site on Monday. (AP PHOTO)
Kim said a total of 102 people were working at the factory before the fire occurred.
Prime Minister Han Duck-soo and Interior and Safety Minister Lee Sang-min visited the site later Monday.
Han asked officials to provide government assistance for funeral services and support programs for victims' relatives, according to Han's office.
