China appealed for "calm and restraint" on the Korean peninsula Wednesday, after North Korea blocked South Korean entry to a key joint industrial zone at Kaesong.

Pyongyang's move was the latest in a cycle of escalating tensions since it fired a long-range rocket in December and conducted its third nuclear test in February.

"Under the current circumstances China believes that all parties should exercise calm and restraint," foreign ministry spokesman Hong Lei told reporters at a regular briefing, repeating Beijing's oft-declared position.

Vice-minister Zhang Yesui had met North Korea's ambassador on Tuesday, and those of South Korea and the United States, to express "serious concern over the situation in the peninsula", added Hong.

China urged that all sides "do not take provocative actions and should not take actions that would worsen the situation", he said. "We hope all parties look at the long term, engage in dialogue and improve their relations."

Beijing has been Pyongyang's sole major ally for decades and is its biggest trading partner, providing key energy supplies to the poverty-stricken nation.

Tensions on the peninsula have soared in over the past week, with Pyongyang announcing it would restart a nuclear reactor to feed its nuclear weapons programme.

On Friday it ordered missile units to standby, hours after nuclear-capable US B-2 stealth bombers were deployed in US joint military drills with South Korea.

Any move on the Seoul-funded Kaesong complex -- set up in 2004 and a vital source of hard currency for North Korea -- carries tremendous significance and the potential to send tensions mounting further.

Neither of the Koreas has allowed previous tensions to significantly affect Kaesong, the only surviving example of inter-Korean cooperation and seen as a bellwether for the stability of the peninsula.

The US on Tuesday called on China to do more to rein in North Korea, with White House spokesman Jay Carney telling reporters: "It is not a mystery to anyone that China has influence with North Korea.

"We have in the past and are now urging China to use that influence to try to affect North Korean behaviour."

N. Korea blocks access to key industrial zone
Seoul (AFP) April 3, 2013 - 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" behaviour.

North Korea informed Seoul in the morning that it was stopping the daily movement of South Koreans into the Seoul-funded Kaesong complex -- 10 kilometres (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 defence 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.

Describing the Kaesong ban as "very regrettable", South Korea's Unification Ministry urged the North to normalise access "immediately".

"Otherwise ... not only will inter-Korean relations be negatively affected but North Korea will invoke greater criticism and isolation from the international community," the ministry said in a statement.

It added that 33 South Koreans had returned from Kaesong, with hundreds staying on to keep their companies running smoothly. Many routinely stay for periods of several days.

Around 53,000 North Koreans work at 120 South Korean plants at the complex, which was still operating normally Wednesday.

Lee Jae-Young, manager at a watchmaking plant in Kaesong, was among those prevented from crossing into the North.

"I feel anxious about my colleagues there. This is an emergency situation and it doesn't look good," Lee told AFP.

"This could also be serious trouble for our business which requires the constant shipping of raw materials to Kaesong for manufacturing," she added.

But Kim Dong-Kyu, a South Korean manager still in Kaesong, was more relaxed.

"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.

Cho Han-Bum, an analyst at the Korea Institute for National Unification, said Pyongyang was unlikely to risk a complete shutdown of Kaesong.

"There are more than 53,000 North Koreans working there, most of whom have immediate and extended family members who depend on them," Cho told AFP.

"Shutting the whole thing down is financially significant enough to cause a riot among these people," he added.

Tensions have been soaring on the Korean peninsula since the North launched a long-range rocket in December and conducted its third nuclear test in February. Both events triggered UN sanctions.

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.

US Secretary of State John Kerry on Tuesday 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 warned it would reopen its mothballed Yongbyon reactor -- its source of weapons-grade plutonium.

Earlier, UN chief 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," Ban 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.