SEOUL -- North Korea blocked access to a key joint industrial zone with South Korea on Wednesday -- a sharp escalation in a military crisis that Washington blamed on Pyongyang's "reckless" behavior.

North Korea informed Seoul in the morning that it was stopping the daily movement of South Koreans into the Seoul-funded Kaesong complex -- six miles inside the North side of the border.

However, it added that the 861 South Koreans currently in the zone were at liberty to leave.

Any move on Kaesong -- established in 2004 and a crucial source of hard currency for North Korea -- carries enormous significance.

Neither of the Koreas has allowed previous crises to significantly affect the complex, which is the only surviving example of inter-Korean cooperation and seen as a bellwether for stability on the Korean peninsula.

South Korea's defense ministry said it had contingency plans that included "military action" in case the safety of its citizens working there was threatened.

The latest North Korean move fitted into a cycle of escalating tensions that has seen Pyongyang threaten missile and nuclear strikes against the United States and its ally South Korea in response to UN sanctions and joint military drills.

China, the North's sole major ally, appealed for "calm" from all sides Wednesday, repeating Beijing's oft-declared position.

"Under the current circumstances China believes that all parties should exercise calm and restraint," foreign ministry spokesman Hong Lei said.

Russia's Deputy Foreign Minister Igor Morgulov said he was "worried" by the situation, saying even a simple human error could cause the crisis to spiral out of control.