SEOUL, South Korea -- North Korea blocked South Koreans from crossing the heavily armed border to a jointly operated industrial park on Wednesday, raising the possibility that the North might be adding to its recent cascade of threats and provocative actions by cutting the last remaining major symbol of inter-Korean cooperation.
Angry over joint American-South Korean military drills and a recent round of United Nations sanctions, the North has in recent weeks threatened to strike at the United States, the South's ally, in Guam, Hawaii and the mainland United States. While analysts doubt the potency of the North's arsenal, Defense Secretary Chuck Hagel responded Wednesday that the North's recent inflammatory language and actions presented "a real and clear danger" to the interests of South Korea, Japan and the United States, and the Pentagon announced that it would significantly increase its missile-defense systems deployed to the Pacific.
A land-based Terminal High Altitude Area Defense system, or THAAD, would be sent to Guam in the coming weeks "as a precautionary move to strengthen our regional defense posture against the North Korean regional ballistic missile threat," the Pentagon said. The system includes a truck-mounted launcher, interceptor missiles, an integrated fire control system and advanced tracking radar.
Earlier this week, the Defense Department announced that two of the Navy's Aegis-class missile-defense warships also would be positioned in Pacific waters to watch North Korea. Those vessels have radar and interceptor missiles, as well. Adding a land-based system to Guam would free the ships to provide coverage to other areas.
The jointly operated industrial complex, in the North Korean town of Kaesong, had continued to operate for days since the North threatened to shut it down. But on Wednesday, more than 480 South Koreans who showed up at a border crossing were denied permission to cross, said the Unification Ministry of South Korea, which is in charge of relations with the North. North Korea promised to allow 861 South Koreans currently staying in Kaesong to return home if they wished, the ministry said. But with no replacements arriving, only 33 immediately decided to return home.
The eight-year-old industrial park, on the western edge of the border of the two Koreas, produced $470 million worth of goods last year, helping provide a badly needed source of hard currency for the cash-strapped North. It generates more than $92 million a year in wages for 53,400 North Koreans employed by 123 textile and other labor-intensive South Korean factories there.
It was not the first time that North Korea had disrupted the park's operation. It blocked cross-border traffic three times in 2009, once for three days, out of anger over joint military drills by South Korean and American troops. That blockade was lifted when the military exercises ended. The current American-South Korean military drills are to continue until the end of April.
China's deputy foreign minister, Zhang Yesui, met with the ambassadors of the two Koreas and the United States on Tuesday to express serious concern over the situation on the Korean Peninsula, Hong Lei, a spokesman of the Chinese Foreign Ministry, said Wednesday.
"The improvement of relationships between the two Koreas, as well as their reconciliation and cooperation, are conducive to the peace and stability on the peninsula," he said. "We hope the two Koreas can resolve the relevant issues through dialogue and consultation."
Meanwhile, the United States and South Korea are entering the final stretch of long negotiations over another highly sensitive nuclear issue: South Korea's own request for American permission to enrich uranium and reprocess spent nuclear fuel.
The request comes at a delicate time. South Korea insists that it needs to produce fuel for its fast-expanding nuclear energy industry and reduce its almost-full nuclear waste storage. But the same technologies can also used to make material for nuclear weapons.
In 1972, when Washington transferred nuclear material, equipment and technical expertise to help build South Korea's nuclear energy industry, it had Seoul commit itself not to enrich or reprocess. That deal expires in March 2014, and both sides are racing to work out a revised and updated version; it has to be submitted to Congress before the summer for approval.
South Korea has lingering American misgivings to dispel. The current president's father, the late military strongman Park Chung-hee, feared being abandoned by the United States when then-President Jimmy Carter talked about withdrawing American troops from South Korea in the 1970s, and tried to build nuclear weapons. Washington got wind of the effort and blocked it. The South's scientists dabbled in reprocessing in 1982 and enrichment in 2000 and failed to declare their activities to the International Atomic Energy Agency.
For Washington and Congress, allowing South Korea to develop either the enrichment or reprocessing technologies would mark a rare exception, one that nonproliferation advocates said would set a bad precedent, undermine Washington's global efforts to curb the spread of such activities and further undermine American efforts to persuade North Korea and Iran to give up its nuclear programs.
Secretary of State John Kerry and his South Korean counterpart, Foreign Minister Yun Byung-se, discussed the long-running South Korean desire in Washington on Tuesday and said they will take it up again when Mr. Kerry visits Seoul next week. Both sides hope for a compromise before President Obama and President Park Geun-hye of South Korea are scheduled to meet in Washington in May. Ms. Park made winning Americans concessions on the issue one of her top campaign pledges for her December election.
"I am very hopeful, and I think the foreign minister shares this hope, that this can be resolved before the visit of President Park," Mr. Kerry said in a joint news conference with Mr. Yun on Tuesday. The South Korean minister called for a "mutually beneficial, timely, and forward-looking" solution.
In South Korea, where people remember their recent history of war and foreign occupation and feel squeezed by bigger countries they consider bullies, popular support has often surged for arming the country with nuclear weapons -- especially when people became doubtful of the American commitment to defend their country or when the North's threats intensify, as they have in recent weeks.
"When the thug in the neighborhood has gotten himself a brand-new machine-gun, we can't defend our home with a stone," Chung Mon-joon, a ruling party leader and vocal champion for "a nuclear sovereignty" for South Korea, recently said, referring to the North Korean nuclear threat. "At a time of crisis, we are not 100 percent sure whether the Americans will cover us with its nuclear umbrella."
But such a call, even if it reflected popular sentiments, has never become a national debate, tamped down by unequivocal rebuttals from government policy-makers. And the United States flew nuclear-capable B-52 and B-2 bombers in recent training sorties over the Korean Peninsula, demonstrating its commitment to a nuclear umbrella for the South Korean ally.
Mark Landler and Thom Shanker contributed reporting from Washington, and Patrick Zuo from Beijing.
This article originally appeared in The New York Times.