Firefighters extinguished the blaze at the sanatorium's annex building in Jangseong, South Korea's South Jeolla province in about 30 minutes, officials said, but the building was still filled with heavy smoke that might have caused asphyxial or poisoning death of the patients, Xinhua quoted Yonhap News Agency as saying in a report.
The nursing home had around 30-40 patients plus employees. About 10 patients on the first floor managed to escape soon after the fire started, but some 30 patients on the second floor failed to do so.
Most of the patients were in their 70s and 80s lying in bed, having little or less mobility, Yonhap reported.
As six wounded are in critical condition, the death toll may rise, police said.
The cause of the fire, which broke out at 00:27 a.m., still remains unknown.
The accident comes a day after seven people were killed and 41 others injured in a fire at a bus terminal in the country's Goyang city.