Pyongyang, April 4: North Korea blocked South Korean access to a key joint industrial zone on Wednesday, matching its angry rhetoric with action as Washington condemned Pyongyang's "dangerous, reckless" behaviour.

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 yesterday 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 this morning that it was stopping the daily movement of South Koreans into Kaesong, which lies 10 kilometers 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 on Wednesday.

Kim Dong-Kyu, a South Korean company manager waiting to leave Kaesong, said 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 Korean air and sea space.