Workers are allowed to leave the Kaesong industrial zone but not cross into it from the South, Seoul's Unification Ministry confirmed.
The Kaesong zone generates $2 billion a year in trade for the impoverished North.
"South Korea's government deeply regrets the entry ban and urges it be lifted immediately," Unification Ministry spokesman Kim Hyung-seok said. "Ensuring the safety of our citizens is our top priority and the South Korean government will take necessary measures based on this principle."
For over a month North Korea has been threatening the United States with a nuclear attack and to bomb its Pacific bases as well as saying it will wage war on South Korea. It has also said it would restart a nuclear reactor that it uses to produce plutonium for its nuclear weapons programme.
Last week Pyongyang threatened to shut down the Kaesong complex last week.
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