But in the latest sign that North Korea's warlike stance toward South Korea and the United States is moving from words to action, the North on Wednesday barred South Korean managers and trucks delivering supplies from crossing the border to enter the Kaesong industrial park.
It's an announcement that further escalates a torrent of actions that analysts say is aimed at pressuring the U.S. and South Korea to change their policies toward North Korea.
The Kaesong move came a day after the North said it would restart its long-shuttered plutonium reactor and a uranium enrichment plant. Both could produce fuel for nuclear weapons that North Korea is developing and has threatened to hurl at the U.S., but which experts don't think it will be able to accomplish for years.
The North's rising rhetoric has been met by a display of U.S. military strength, including flights of nuclear-capable bombers and stealth jets at annual South Korean-U.S. military drills that the allies call routine and North Korea says are invasion preparations.
The Kaesong industrial park started producing goods in 2004 and has been an unusual point of cooperation in an otherwise hostile relationship between the Koreas, whose three-year war ended in 1953 with an armistice, not a peace treaty.
It has remained open despite the sinking of a South Korean warship in 2010, killing 46 people, which Seoul blamed on the North, and a North Korean artillery attack on a front-line South Korean island later that year that killed four people. The North denies involvement in the ship sinking and says a South Korean live-fire drill triggered the bombardment.
Kaesong's continued operation through those episodes of high tension, and its high economic value to impoverished North Korea, has reassured foreign multinationals that another Korean War is unlikely and their investments in prosperous, dynamic South Korea are safe.
"What we are seeing right now is something that was less expected, that is, less directly in North Korea's interests," said Patrick Cronin, a senior analyst with the Washington-based Center for a New American Security. "Is this a short-term demonstration of North Korean dissatisfaction with U.S-South Korean policy, or a portent of something more drastic at Kaesong?" he said.
Drastic could range from a complete shutdown of Kaesong to North Korea taking South Korean workers at the facility hostage, which is a risk that has long hung over the joint project, Cronin said.
On Tuesday, a senior South Korean government official said Seoul has a contingency plan for its citizens in Kaesong, which number over 800 on weekdays. Most South Korean managers at Kaesong return to South Korea on the weekends. The official spoke on condition of anonymity, saying he was not authorized to speak publicly to the media.
It is unclear how long North Korea will prevent South Koreans from entering the industrial park, which is located in the grim North Korean border city of Kaesong and provides jobs for more than 50,000 North Koreans who make goods such as textiles, clothing and electronic components. The last major disruption at the park amid tensions over U.S.-South Korean military drills in 2009 lasted just three days.
In Washington, Defense Secretary Chuck Hagel said in a speech at the National Defense University that the north's rhetoric represents a "real, clear" danger to the U.S. and its Asia-Pacific allies and said America is doing all it can to defuse the situation.
State Department spokeswoman Victoria Nuland said the North's move was "regrettable" and should be lifted. She said it only works to the detriment of North Korea, given the number of its citizens employed at Kaesong.
"So again, this is just a choice that further isolates the country rather than taking them in the direction of a better future for their people," Nuland told reporters.