On Thursday, North Korea barred South Korean workers and managers for a second day from entering the Kaesong industrial complex, an economic cooperation zone that sits on the North's side of the border but houses operations of scores of South Korean companies.
It also repeated a threat from the weekend to completely shut down the complex, where more than 50,000 North Koreans currently work.
The current crisis at Kaesong began a day after North Korea said it planned to restart "without delay" a reactor at its main nuclear complex that it had shut down five years ago as part of a deal with the United States, China and four other nations.
Most observers say the North is still years away from having the technology to deliver a nuclear warhead on a missile.
It has conducted three nuclear bomb tests, in 2006, 2009 and most recently in February. It has said that its nuclear weapons are a deterrent and are no longer up for negotiation.
But U.S. officials have said they see no unusual military movements across the Demilitarized Zone that splits the Korean Peninsula.
Many analysts say the increasingly belligerent talk is aimed at cementing the domestic authority of Kim Jong Un.