A patient suffering from Alzheimer's has been arrested after 21 people died in a fire at a hospital in South Korea.

Seven people were injured in the blaze, which broke out at the hospice in Jangseong county about 300km south of Seoul.

Most of the patients who died at Hyosarang Hospital were in their seventies and eighties.

A police official said an 81-year-old man had been taken into custody after CCTV footage appeared to show him setting the fire in a storage room on the upper floor of the two-storey building.

Although the blaze was brought under control within 30 minutes, many on the upper floor were unable to escape.

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

Lee Hyung-Seok, chief administrator of the Jangseong hospice, said: "I'm sorry. I apologise for this terrible thing."

It was South Korea's second deadly blaze in two days. Seven people were killed and 41 injured in a fire at a bus terminal near Seoul on Monday.