At least nine foreign migrants including eight Chinese were killed and 18 others hurt in a blaze early on Sunday at a detention centre in the South Korean port of Yeosu, local officials said.
One Uzbek national was also among the dead in Yeosu, located 450 kilometres (280 miles) south of the capital Seoul.
"The death toll is likely to rise, as 17 of the 18 hurt are in serious condition. They are being treated in hospital for smoke inhalation and burns," an immigration official said by phone.
A total of 55 migrants were in custody at the four-storey building at the time of the pre-dawn blaze. Twenty-eight were rescued by fire fighters.
Kim Dae-Joong, head of the Yeosu fire station, told journalists that many of the victims had died from inhaling toxic fumes.
The cause of the blaze was not immediately known but police were investigating the possibility of arson, the Yonhap news agency reported.
"We don't know whether the fire started because of electric short circuits or arson," the immigration official said.
The fire was put out about one hour after it broke out, with 120 firefighters and 27 trucks rushing to the scene.
The 55 foreign migrants -- 51 men and four women -- had been detained at the facility after being caught trying to illegally enter South Korea.
The group included 42 Chinese, four from Uzbekistan, two from Kazahkstan, two from Sri Lanka, and one each from India, Iran, Kyrgyzstan, Russia and Vietnam.
Lee Bok-Nam, head of the immigration office in Yeosu, told journalists that the facility was manned by nine people including security guards at the time of the fire.
"They were unable to put out the flames in the first minutes of the blaze because of the iron bars on the window," he said.
He said his office would soon contact relatives of the victims to discuss compensation.
Prime Minister Han Myeong-Sook expressed regret over the deaths and ordered a thorough probe into the case, her office said.
"The Prime Minister instructed Justice Minister Kim Sung-Ho to examine all detention facilities for illegal immigrants and work out measures to prevent any recurrence," it said.