South Korean President Park Geun-hye expressed shock Friday at allegations that two disabled men were lured to a salt farm on a remote island and forced into slave-like labor for years.

"They say 'stranger than fiction,' but who could have imagined that such a thing could happen?" Park said in a government meeting. She called for an investigation into labor practices on small islands, of which there are hundreds. In the past decade South Korean media have reported several cases of alleged slavery in the area.

Police in Seoul's Guro district announced last week that they had rescued a 40-year-old visually impaired man surnamed Kim and a 48-year-old mentally disabled man surnamed Chae last month from a salt farm on remote Sinui Island off the country's southwestern coast. Police said they learned of the case when, after several failed escape attempts, Kim managed to secretly send a letter to his mother in Seoul asking to be rescued.

Kim and Chae were forced to work on the salt farm and do other chores without pay and had less than five hours of sleep a day, police said in a statement. It said Kim and Chae, who were registered as missing, were deceived with false promises by job agents in 2012 and 2008, respectively. A salt farm owner surnamed Hong paid 1 million won ($943) for Kim and 300,000 won ($283) for Chae, said Nam Wang-seok, head of the missing persons investigations team at the Guro Police Office.

Nam said police are still collecting evidence. A police officer at a station with jurisdiction over Sinui Island declined to provide information about the case.

According to South Korean broadcaster KBS, police in 2011 arrested a group of people who allegedly kidnapped and sold disabled or homeless people to salt farms for about 1.5 million won ($1,416) per person.

South Korea is one of Asia's top economies and Seoul is an ultra-modern city with large pockets of wealth. But many remote islands have yet to catch up with Seoul in their development.