A fire at a rural South Korean hospital for chronically ill elderly patients has left 21 people dead.

Another eight people were injured in the midnight blaze at the hospital in the southwest region of Jeolla.

The fire was put out relatively quickly, but most of the victims were elderly patients unable to walk or move freely, leading to the large number of casualties, fire officials said.

An 81-year-old man is in police custody and is being questioned on suspicion of arson.

A police official said he was seen on surveillance camera walking into an empty room where the fire is believed to have started.

The man has been identified as a patient at the hospital and is suffering from dementia, the official said.

All the victims were on the second floor of one building, and most of them suffered smoke inhalation, a local fire department official said.

The hospital held patients who required long-term care, many with dementia or disability as a result of a stroke, local media reports said.

South Korea has developed into a vibrant and technically advanced democracy, but it faces criticism that regulatory controls and safety standards have not kept pace.

The country is mourning the deaths of more than 300 people who drowned when a heavily overloaded ferry capsized and sank on 16 April.

It was the country's worst maritime disaster in 20 years.

A fire at a large shopping centre complex killed eight people on Monday, when smoke and toxic fumes spread rapidly.