North Korea's neighbors on missile watch ahead of Kerry visit to Seoul

(CNN) -- North Korea's neighbors continued to keep watch Thursday for a possible missile launch by the secretive regime, a day before U.S. Secretary of State John Kerry was due to arrive in the region.

Amid the tension, the North repeated a threat to permanently close the industrial zone it jointly operates with the South, accusing South Korean President Park Geun-hye of putting the manufacturing complex, a key symbol of inter-Korean cooperation, at risk.

The South Korean government, meanwhile, urged Pyongyang to work to resolve the situation through dialogue.

The temporary shutdown of the complex by the North "is not beneficial to the future of the Korean people," South Korean Unification Minister Ryoo Kihl-jae said. "It caused pain to the companies and the workers."In a sign of the strained relations in the region, North Korea has pulled its more than 50,000 workers out of the Kaesong complex, which is on the northern side of the heavily fortified border that divides the two Koreas, and blocked personnel and supply trucks from entering it from South Korea.

More than 120 South Korean companies have operations there.

In a statement reported Thursday by state-run media, the North Korean government said that what happens at the complex in the coming days "entirely depends on the attitude of the South Korean authorities."

Uneasy calm in the region

The difficulties at the industrial zone are one of the few tangible signs of the tensions between the two sides on the Korean Peninsula.

The U.S. and South Korean militaries are monitoring the movements of mobile ballistic missiles on the east coast of North Korea. And Japan has deployed defense systems, as it had done before North Korean launches in the past, in case any test-fired missile flies near its territory.

But despite the North's barrage of recent threats, which have included warnings to foreigners on the peninsula about their safety in the event of conflict, life is generally continuing as normal in the region.

South Koreans, who have experienced decades of North Korean bombast -- and occasional localized attacks -- have gone about their daily business without alarm.

"South Korea has been living under such threats from the past, and we are always prepared for it," Ryoo, the unification minister, told CNN on Wednesday. He called the current climate "a very ordinary situation."

Tourist visits to the North appear not to have been significantly affected by the situation. China says that while some tour groups have canceled trips, the border between the two countries is still operating normally.Foreign athletes are expected to compete in a marathon Sunday in Pyongyang, one of many sporting events organized by North Korean authorities to celebrate the 101st anniversary next week of the birth of Kim Il Sung, the founder of North Korea and the grandfather of Kim Jong Un.

"Our group just boarded a full plane for #Pyongyang," Uri Tours, a U.S.-based travel agency that arranges trips to North Korea, tweeted late Wednesday. "Mix of tourists and marathon runners on their way to #NKorea."

In a report that diminished the idea of a nation on the brink of war, the state-run Korean Central News Agency said this week that "the ongoing sports tournaments make the country seethe with holiday atmosphere."

But the United States and its allies say that although much of the North's fiery rhetoric is bluster, they are still taking its threats seriously.

'A dangerous line'

U.S. Defense Secretary Chuck Hagel said Wednesday that North Korea was "skating very close to a dangerous line" after weeks of saber rattling.

"Their actions and their words have not helped defuse a combustible situation," Hagel told reporters at the Pentagon. He said the United States and its allies want to see North Korean rhetoric "ratcheted down," but if that doesn't happen, "our country is fully prepared to deal with any contingency."

American radar and satellites are trained on the east coast of the Korean Peninsula, where North Korea is believed to have prepared the mobile missiles for launch at any time, U.S. and South Korean officials have warned.

Amid the uneasy situation, Kerry will arrive in Seoul from London on Friday to meet with South Korean Foreign Minister Yun Byung-se.

After South Korea, Kerry will visit China and Japan, both countries with large stakes in the Korean crisis.Provocative acts

Since December, North Korea has put a satellite in orbit atop a long-range rocket; conducted a nuclear bomb test, its third since 2006; and claimed to be prepared for pre-emptive nuclear attacks on the United States, though most analysts believe it does not yet have that capability.

Its most recent nuclear test, in February, resulted in tougher U.N. sanctions, which infuriated Pyongyang, prompting it to sharpen its threats.

Annual military exercises in South Korea by U.S. and South Korean troops, which often upset the North, have added to the tensions, especially when the United States drew attention to shows of strength such as a practice mission by B-2 stealth bombers.

The U.S. military has since dialed back its displays of force, but the question is whether North Korea will now carry out one of its own.

The North has given ample warning to the world before long-range rocket launches it has conducted in the past, but it is keeping everyone guessing about what it might do this time around.

Multiple launches?

Intelligence suggests that North Korea may be planning "multiple missile launches" in the coming days beyond two Musudan mobile missiles it has placed along its east coast, Pentagon officials told CNN. The officials did not have specifics on the numbers of other missiles and launchers.

One official said the North Koreans are military "masters of deception" and may have planned all along to focus the world's attention on the Musudans while they plan to launch other missiles. That's a tactic they have used in the past, the official said.

The United States is less troubled about the other missiles, a second Pentagon official told CNN.

"We've been seeing some launchers moving around. These are smaller and don't cause us as much concerns," that official said. "We think these movements are within seasonal norms for their exercises."

But he didn't discount the possibility that they might launch some of those, as they often do.

Any launch could take place without the standard notice to commercial aviation and maritime shipping that would warn planes and vessels to stay away from the missile's path, a U.S. official warned earlier this week.

The Musudan is an untested weapon that South Korea says has a range as far as 3,500 kilometers (2,175 miles). That would mean it could reach as far as Guam, a Western Pacific territory that is home to U.S. naval and air bases, and where the United States recently said it was placing missile defense systems.

Authorities in Guam raised the threat level Wednesday to yellow, indicating "a medium risk" for the island.

After a launch, U.S. satellites and radars in the region would be able to calculate the trajectory of missiles within minutes and quickly conclude whether they are on a test path headed for open ocean or potentially headed for land areas such as Japan.

The United States and Japan would then have to decide whether to try to shoot the missiles down, U.S. officials say.