A blaze has swept through a hospital in South Korea, killing 21 people and leaving six others in a critical condition, according to the country's news agency.

Most of the patients who died at the hospital on Wednesday in Janseong county, south of  the capital Seoul, were in their seventies and eighties, Yonhap news agency said, citing police and fire officials.

Thirty bedridden patients were trapped by "choking fumes" on the second floor of an annex building, while 10 patients on the first floor managed to escape, the report said.

One nurse was on duty at the time and was among the dead. 

The fire was extinguished in around 30 minutes, Yonhap cited fire official Kim Kyong-man as saying, adding that officials said the toxic fumes were to blame for the heavy death toll.

The elderly patients were suffering from a range of illnesses including Alzheimer's disease, while others had suffered strokes.

It was the second deadly blaze in South Korea in two days after seven people were killed and 41 injured in a fire at a bus terminal near Seoul on Monday, the AFP news agency reported.

That fire was believed to have been sparked by welding work at an underground restaurant.

The country is still reeling from the loss of around 300 people - mostly schoolchildren - in a ferry disaster last month.

Source: AFP