Eighteen people died and dozens were injured in a fire that engulfed a commercial building in South Korea's Jecheon city on Thursday, firefighters said.

The fire started in the afternoon from a vehicle in the basement parking lot of an eight-storey building in the city, 168 km southeast of Seoul. The building houses a fitness centre, restaurants and a public sauna, Yonhap news agency reported.

Sixteen of the victims, including a 50-year-old woman, were found dead, trapped in the sauna on the second floor, the authorities said.

Another group of people were moved to a hospital after inhaling smoke but they were reportedly not in critical condition.

Some 20 others were rescued from the rooftop where they took refuge from the fire. About 50 fire engines as well as 60 firefighters were sent to the scene along with two helicopters.

The fire was largely put out and firefighters started to search the inside of the building but unusually heavy smoke and toxic gas from the fire was delaying the rescue operation, the authorities said.

Officials said the death toll could increase.