Low-income tenants often live in buildings with tiny single-bed studios once favoured by applicants preparing for various state examinations and known as goshiwon, or exam centres.

The dead and injured were mostly casual labourers or street vendors in their 40s to 60s, living in the dilapidated building, which had no sprinklers, Yonhap news agency said.

"I heard screams and went out. Then I saw the building enveloped by a lot of smoke and flames," a 61-year-old businessman living across the street told Yonhap.

With labyrinthine structures with narrow corridors, goshiwon buildings are notoriously vulnerable to blazes, with more than 250 breaking out over the past five years.