Witnesses saw firefighters taking up to six bodies out of the factory. Due to the intensity of the blaze, rescuers were finding it difficult to identify the dead, Mr Kim added.
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 an explosion.
Aerial footage showed massive smoke clouds billowing from the structure.
Yoon Suk Yeol, South Korea’s president, was monitoring the situation, his office said, while Lee Sang-min, the interior minister, called on the local authorities to take steps to prevent any hazardous chemicals from contaminating the surroundings.
Authorities in Hwaseong sent out a series of alerts to residents warning them to stay inside.
“There is a lot of smoke due to factory fires. Please pay attention to safety, such as refraining from going out,” one alert sent by text message said.
“Factory fire. Please detour to surrounding roads and nearby citizens please close windows,” another one read.
The lithium battery plant is owned by Aricell, a South Korean primary battery manufacturer.
Lithium batteries are used in everything from laptops to electric vehicles – but can be highly explosive, with airlines, for example, imposing strict regulations on checking devices containing them.