SEOUL - North Korea blocked South Korean access to a key joint industrial zone Wednesday, matching its angry rhetoric with action as Washington condemned Pyongyang's "dangerous, reckless" behaviour.
A South Korean soldier sets up a military check point on the road to the Kaesong industrial complex in North Korea, in this photo taken on Monday.
Any move on the Seoul-funded Kaesong complex - established in 2004 and a crucial source of hard currency for North Korea - carries enormous significance and will send tensions soaring.
Neither of the Koreas have allowed previous crises to significantly affect Kaesong, the only surviving example of inter-Korean cooperation and seen as a bellwether for the stability of the Korean peninsula.
The latest North Korean move fitted into a cycle of escalating tensions that prompted UN chief Ban Ki-Moon to warn Tuesday that the situation had "gone too far" as the US vowed to defend itself and regional ally South Korea.
South Korean officials said the North had informed them Wednesday morning that it was stopping the daily movement of South Koreans into Kaesong, which lies 10 kilometers (six miles) on the North side of the border.
However, it would allow the 861 South Koreans currently in Kaesong to leave, they said.
Describing the North's move as "very regrettable", Unification Ministry spokesman Kim Hyung-Suk called on the North to normalise movement to Kaesong "immediately."
Kim said the North had not specified how long the access ban would remain in effect.
Around 53,000 North Koreans work at plants for 120 South Korean firms at the complex, which was still operating normally Wednesday.
Kim Dong-Kyu, a South Korean company manager waiting to leave Kaesong, told the YTN news channel that he wasn't "particularly" worried.
"Plants are operating normally and the atmosphere here is like, business as usual. It doesn't appear that the complex will be closed as far as I can tell," Kim said.
The last time the border crossing was blocked was March 2009 in protest at a major US-South Korean military exercise. It reopened a day later.
Tensions have been soaring on the Korean peninsula since the North held a nuclear test in February, having launched a long-range rocket in December.
Infuriated by subsequent UN sanctions, the North has threatened the United States and South with everything from artillery attacks to nuclear strikes.
In a rare show of force in the region, Washington has deployed nuclear-capable US B-52s, B-2 stealth bombers and two US destroyers to South Korea.
US Secretary of State John Kerry held talks in Washington Tuesday with his South Korean counterpart Yun Byung-Se, after which he denounced the "unacceptable rhetoric" emanating from Pyongyang and North Korean leader Kim Jong-Un.
"What Kim Jong Un has been choosing to do is provocative. It is dangerous, reckless and the United States will not accept (North Korea) as a nuclear state," Kerry said.
He was speaking after the North triggered renewed alarm by warning it would reopen its mothballed Yongbyon reactor -- its source of weapons-grade plutonium.
"Let me be perfectly clear here today. The United States will defend and protect ourselves and our treaty ally, the Republic of Korea," Kerry said.
Earlier, Ban Ki-moon warned the situation was veering out of control and stressed that "nuclear threats are not a game."
"The current crisis has already gone too far... Things must begin to calm down," the former South Korean foreign minister said, adding that negotiations were the only viable way forward.
The North shut down the Yongbyon reactor in July 2007 under a six-nation aid-for-disarmament accord.
Experts say it would take at least six months to get the reactor back up and running, after which it would be able to produce one bomb's worth of weapons-grade plutonium a year.
Many observers believe the North has been producing highly-enriched uranium in secret facilities for years, and that the third nuclear test it conducted in February may have been of a uranium bomb.
Its previous tests in 2006 and 2009 were both of plutonium devices.