North Korea said it would only allow South Koreans staying at the industrial complex in Kaesong, just north of the Demilitarized Zone, to go return to the South, said Kim Hyung-suk, a spokesman for the South Korean Unification Ministry.
The sudden entry ban on South Korea workers came a day after the North announced it would resume operations at its Yongbyon nuclear complex in violation of international commitments.
The North also, in response to tightened U.N. Security Council sanctions and to ongoing U.S.-South Korea annual military drills, threatened to launch pre-emptive nuclear attacks on the United States and South Korea.
The South Korean Unification Ministry spokesman said the agency was informed Wednesday by the North about its decision on the Kaesong workers, Yonhap News reported.
Earlier in the day, the North stopped issuing daily permits to South Korean managers and cargo trucks at the DMZ, leaving several workers to wait for hours before going back.
"South Korea's government deeply regrets the entry ban and urges it to be lifted immediately," Kim said. He said the government will ensure the safety of South Korean nationals at the industrial site.
The complex was created in 2000 under an inter-Korean agreement and remains the only economic link between the two Koreas.
The two sides' combined output is reported to be more than $2 billion.
"The government will talk with companies that have factories at Kaesong to determine what course of action should be taken," Kim said.
Yonhap said there were 861 South Koreans at the Kaesong complex before the North announced its decision. Since then 17 of them had returned to the South.
Separately, the Kaesong Industrial District Management Committee informed Seoul that South Korean plants at the complex were operating normally, Yonhap reported.
Such actions by the North have occurred in the past.
A lawmaker from the South Korean ruling party quoted South Korean Defense Minister Kim Kwan-jin as telling a special meeting that his ministry would take military action if the safety of South Koreans at the Kaesong complex is threatened, Yonhap reported.