North Korea on Wednesday barred South Koreans from entering a key Seoul-funded joint industrial zone in Kaesong along the North's side of the border and warned that it had finalized plans for "merciless" military strikes on the United States.
The moves marked an escalation in nuclear-armed North Korea's month-long standoff with South Korea and ally the United States.
They came a day after the North said it would restart its long-shuttered Yongbyon nuclear reactor, the latest in a catalog of actions by young leader Kim Jong Un's regime since it fired a long-range rocket in December and conducted its third nuclear test in February, drawing international sanctions, including from ally China.
Set up in 2004, the Kaesong Industrial Park is a key revenue earner for cash-starved North Korea, and has 120 South Korean plants employing more than 50,000 North Koreans. It is about 10 kilometers (six miles) inside the north side of the heavily-fortified border.
Both the two Koreas have not allowed previous crises to stifle the park's operations, seen as a living example of cooperation between the two countries, which have technically been at war for the last 60 years, as the 1950-53 Korean War ended with an armistice rather than a formal peace treaty.
Last month, however, North Korea said that it would no longer be bound by the truce, in protest against South Korea’s joint military exercises with the United States, which has deployed nuclear-capable B-2 stealth bombers and F-22 stealth fighters and two Aegis anti-missile destroyers to South Korean air and sea space.
South Korean officials said North Korea is allowing South Koreans to return home from Kaesong but about 480 South Koreans who had planned to travel to the park Wednesday were refused entry.
Some 33 workers out of about 860 South Koreans at Kaesong returned Wednesday.
In a meeting with lawmakers, South Korean Defense Minister Kim Kwan-jin said he was looking at all available options to ensure the safety of South Koreans who remain in the industrial zone, including military action in a worst-case scenario, the South's Yonhap news agency reported.
Although officials in Seoul believe chances are slim that Pyongyang would hold South Korean workers hostage, military and government officials have annually practiced various scenarios of hostage rescue missions during annual computer-aided drills, Yonhap quoted military officials as saying.
In a related development, Pyongyang said it had "ratified" a merciless attack against the United States, potentially involving a "diversified nuclear strike," according to a statement published by the state KCNA news agency, Reuters reported.
"We formally inform the White House and Pentagon that the ever-escalating U.S. hostile policy toward the DPRK (North Korea) and its reckless nuclear threat will be smashed by the strong will of all the united service personnel and people and cutting-edge smaller, lighter and diversified nuclear strike means of the DPRK and that the merciless operation of its revolutionary armed forces in this regard has been finally examined and ratified," a spokesman for the General Staff of the Korean People's Army said in a statement.
The North had last month threatened a "pre-emptive" nuclear strike on the United States, and last week its supreme army command ordered strategic rocket units to combat status.
In Washington, Defense Secretary Chuck Hagel said that the North's rhetoric represents a "real, clear" danger to the United States and its Asia-Pacific allies, adding that the U.S. is doing all it can to defuse the situation.
"We take those threats seriously, we have to take those threats seriously," he said. "We are doing everything we can, working with the Chinese and others to defuse that situation on the [Korean] peninsula."
"I hope the North will ratchet its very dangerous rhetoric down."
China, North Korea'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, according to Agence France-Presse.
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, AFP reported.
Reported by RFA's Korean Service. Written in English by Parameswaran Ponnudurai.