S. Korean Workers Denied Access to Joint Industrial Complex

South Korean vehicles turn back after they were refused for entry to North Korea's city of Kaesong, at the customs, immigration and quarantine office in Paju, South Korea, near the border village of Panmunjom, April 3, 2013.

SEOUL — North Korea on Wednesday suspended entry of South Koreans workers into a joint industrial zone just north of the border. It is the latest sign of rapidly escalating tension on the peninsula and puts at risk one of the last remaining signs of cooperation between the two foes.

Hundreds of South Korean managers commute to the joint factory park, just north of the fortified border separating the two Koreas. But those who tried to ride into the Kaesong Industrial complex Wednesday morning found they were denied entry permits by the North Koreans.

Unification Ministry spokesman Kim Hyun-suk in Seoul expressed strong regret about North Korea's action. He said blocking access will have ramifications, if supplies and food cannot be replenished.

Kim says this disruption poses a “serious obstacle to the proper operation” of the complex.

Of the approximately 800 South Koreans who had stayed overnight in the zone, about 50 were expected to leave Wednesday, with the rest choosing to stay there, for now.

There has been concern that, if hostilities were to erupt between the two countries, any South Koreans at Kaesong could be potential hostages.

Wednesday, South Korea's defense minister told governing party members of the National Assembly that a “contingency plan, including possible military action,” should be developed should there be such a serious situation.

Although about 125 South Korean companies have factories there, the unique project, which has been producing household goods since 2004, is of greater economic value to North Korea.

Fifty-thousand of its factory workers are North Koreans and the complex brings in $2 billion annually of desperately needed hard currency for the impoverished and isolated country.

North Korea's move to bar, at least temporarily, the entry of South Koreans to Kaesong comes a day after Pyongyang announced it would restart operations at its Yongbyon reactor complex to make additional nuclear weapons.

North Korea is under United Nations sanctions for its nuclear and ballistic missile development.

In recent weeks, North Korea has made a series of bellicose declarations. It has renounced the 1953 armistice, vowed a preemptive nuclear strike on the United States and South Korea and declared a state of war between the North and South.

General James Thurman, commander of the 28,000 U.S. forces in the South, told ABC News at the Joint Security Area inside the demilitarized zone the situation is as tense as any time since he assumed command in mid-2011.

“The situation is volatile and it is dangerous," he said.

When asked his greatest fear with Kim Jong Un, General Thurman stated "a miscalculation and an impulsive decision that causes a kinetic provocation.”

The general also heads the U.S.-led United Nations command and would be in charge of South Korean forces under a unified command, should all-out war begin.

The two Koreas faced each other for three years during a war in the early 1950's that devastated the peninsula. Open hostilities ended with an armistice signed by North Korea, China and the U.S.-led UN command, representing 16 countries. South Korea was not a signatory.