South Korea has suspended the admission of guest laborers from Vietnam because too many of them have disappeared after their visas expired.

Officials said Wednesday that Seoul has stopped allowing Vietnamese to take a Korean language test which is required before temporary workers can be admitted.

More than 4,000 Vietnamese workers entered South Korea in 2004 and 2005 under a program that allows them to stay for up to six years. But only about half of those went home at the end of the six years.

South Korean officials say they will not resume offering the test until the number of illegals comes down.

South Korea has admitted tens of thousands of foreign workers — including about 60,000 Vietnamese — under the guest worker program. The scheme is seen as economically essential because many South Koreans refuse to take dirty or dangerous jobs.

The program is also important for Vietnam, which sends hundreds of thousands of workers abroad. The World Bank estimates that remittances from overseas workers account for 7 percent of Vietnam’s gross domestic product.