Concern is rising over foreigners' misuse of Jeju Island's visa waiver program to stay in Korea illegally, as more than 4,300 people arriving on the resort island disappeared last year alone.
They are suspected of staying there illegally, or crossing over to the mainland to seek work.
On Jeju, visas are waivered for nationals of all but 11 countries, for up to 30 days.
The number of foreigners coming to Jeju on the visa waiver program is on a steep rise, from 232,000 in 2012 to 429,900 in 2013, 645,300 in 2014 and 629,700 last year.
Also on the rise is the number of such visitors that go missing: 371 in 2012, 731 in 2013, 1,450 in 2014 and 4,353 last year.
In the most recent case, a group of 155 Vietnamese travelers came to Jeju on a six-day tourism program on Jan. 12. But 56 of them disappeared. As of Monday, only 28 have been located, according to the Jeju branch of the Korea Immigration Service.
While most of them were hiding at other lodgings, three found jobs at food factories in Hallim District after being illegally hired as cheap labor.
Immigration authorities are tracking down a suspected broker who allegedly helped the three get jobs there in return for $15,000 to $45,000 per person.
Immigration officers said the factory owner hired the three without knowing they had run away from the tour program and does not know where the broker is.
Authorities said they plan to beef up security at the island's airport and ports to prevent visitors from traveling to the mainland illegally. They plan to increase crackdowns on brokers ― mostly from China ― as well.
"Most of the foreigners staying here illegally seek employment at local factories or construction sites," said an official of the Jeju Immigration Office. "After a few years of experience, they move to bigger companies for better payment."
He said that travel agencies are required to report missing tourists to the immigration authorities only if they have a tourist visa. "But for those coming without a visa, the agencies are not required to do so. The visa waiver program needs to be revised to make such reports mandatory."