(CNN) -- North Korea kept tensions simmering around its borders Thursday, reportedly moving a medium-range missile to its east coast and continuing to put pressure on a joint industrial complex where hundreds of South Koreans work.
The day before, the United States had announced it was sending ballistic missile defenses to Guam, a Western Pacific territory that's home to U.S. naval and air bases. North Korea has cited those bases among possible targets for missile attacks.
South Korean Defense Minister Kim Kwan-jin told a parliamentary committee in Seoul that the North has moved a medium-range missile to its east coast for an imminent test firing or military drill. The missile doesn't appear to be aimed at the U.S. mainland, Kim said, according to the semi-official South Korean news agency Yonhap.
The movement of the missile is "of concern, certainly to the U.S. military and to Japan," said Mark Fitzpatrick, director of the Non-Proliferation and Disarmament Program at the International Institute for Strategic Studies in London.
He said he believed the missile in question was a Musudan, a missile the North hasn't tested before that is based on a Soviet system with a range of about 2,400 kilometers (1,500 miles), far enough to reach Japan but not Guam.
The U.S. military, which has a string of bases and thousands of troops in Japan, has already moved two warships and a sea-based radar platform near to the Korean Peninsula to monitor possible missile activity, U.S. defense officials said earlier this week.
"The concerning development is if they test a Musudan and it works, then they have a new proven system that could reach anywhere in Japan," Fitzpatrick said.
Another worry is that the missile's test flight could pass over Japan, straining nerves in an already jittery region.
North Korea isn't believed to have an operational missile that can reach the U.S. mainland at the moment.
The medium-range missile will probably take about two weeks to prepare, Fitzpatrick said, which means a potential launch could coincide with the April 15 anniversary of the the birth of Kim Il Sung, the founder of North Korea and grandfather of its current leader, Kim Jong Un.
Known as "the Day of the Sun," Kim Il Sung's birthday is a major public holiday in North Korea that is usually accompanied by large-scale parades.
A fresh burst of rhetoric
The reported missile activity Thursday followed Pyongyang's latest salvo of ominous rhetoric, which revived the alarming but improbable threat of a nuclear attack against the United States and warned that "the moment of explosion is approaching fast."
The fraught situation on the Korean Peninsula stems from the North's latest long-range rocket launch in December and underground nuclear test in February.
The tougher U.N. sanctions in response to those moves, combined with joint U.S.-South Korean military exercises in the region, have prompted the regime of Kim Jong Un to ratchet up its threats in recent weeks.
The United States has in turn made a show of its military strength in the annual drills taking place at the moment, flying B-2 stealth bombers capable of carrying conventional or nuclear weapons, Cold War-era B-52s and F-22 Raptor stealth fighters over South Korea.
But those actions have provided fresh material for Pyongyang's rhetorical outbursts, which have portrayed the practice flights as threats against North Korea.
"The moment of explosion is approaching fast. No one can say a war will break out in Korea or not and whether it will break out today or tomorrow," a spokesman for the General Staff of the North's Korean People's Army (KPA) said early Thursday.
"The responsibility for this grave situation entirely rests with the U.S. administration and military warmongers keen to encroach upon the DPRK's sovereignty and bring down its dignified social system with brigandish logic," the KPA spokesman said in a statement published by the state-run Korean Central News Agency (KCNA).
DPRK is short for Democratic People's Republic of Korea, the official name for North Korea.
Doubts over nuclear capabilities
Most observers say the North is still years away from having the technology to deliver a nuclear warhead on a missile.
U.S. officials have said they see no unusual military movements across the Demilitarized Zone that splits the Korean Peninsula, despite weeks of bombastic rhetoric from Pyongyang.
And many analysts say the increasingly belligerent talk is aimed at cementing the domestic authority of the country's young leader, Kim Jong Un.
But the North does have plenty of conventional military firepower, including medium-range ballistic missiles that can carry high explosives for hundreds of miles. And U.S. Defense Secretary Chuck Hagel said Wednesday that the recent North Korean threats to Guam, Hawaii and the U.S. mainland have to be taken seriously.
"It only takes being wrong once, and I don't want to be the secretary of defense who was wrong once," Hagel told an audience at Washington's National Defense University.
But Hagel also said there was still a "responsible" path for the North to take.
"I hope the North will ratchet this very dangerous rhetoric down," Hagel said. "There is a pathway that is responsible for the North to get on a path to peace working with their neighbors. There are many, many benefits to their people that could come. But they have got to be a responsible member of the world community, and you don't achieve that responsibility and peace and prosperity by making nuclear threats and taking very provocative actions."
For the time being, Pyongyang is showing little interest in taking that path.
Tensions at the border
On Thursday, it barred South Korean workers and managers for a second day from entering the Kaesong industrial complex, an economic cooperation zone that sits on the North's side of the border but houses operations of scores of South Korean companies.
It also repeated a threat from the weekend to completely shut down the complex, where more than 50,000 North Koreans currently work.
"If South Korea's puppet government and conservative media continue to say bad things and make noise, we will take firm action and pull out all of our workers from the Kaesong industrial complex," a spokesman for the North's Committee for the Peaceful Reunification of Korea said, according to KCNA.
The pressure Pyongyang is putting on the companies in Kaesong is significant because the zone is considered to be an important source of hard currency for Kim's regime.
Analysts have said they think it is unlikely the North would go as far as shuttering the complex entirely, since it would be harming itself more than the South.
More than 800 South Koreans remained inside Kaesong on Thursday morning, the South Korean government said, with some staying longer than usual to compensate for those the North is preventing from entering from across the border.
Pyongyang so far isn't stopping South Koreans from leaving the complex.
The North has blocked the crossing into Kaesong before -- in March 2009, another period when joint U.S.-South Korean military drills had upset it. It returned the situation to normal after about a week.
The current crisis at Kaesong began a day after North Korea said it planned to restart "without delay" a reactor at its main nuclear complex that it had shut down five years ago as part of a deal with the United States, China and four other nations.
Threats 'down the road'
China, a key North Korean ally, has expressed frustration with some of Pyongyang's recent moves and has repeatedly called on all parties concerned to exercise restraint.
But the North Korean military's statement Thursday didn't suggest Beijing's comments were holding much sway.
"We formally inform the White House and Pentagon that the ever-escalating U.S. hostile policy toward the DPRK 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," it said. "The U.S. had better ponder over the prevailing grave situation."
Pyongyang had already threatened the possibility of a pre-emptive nuclear strike against the United States and South Korea in March.
The North has conducted three nuclear bomb tests, in 2006, 2009 and most recently in February. It has said that its nuclear weapons are a deterrent that are no longer up for negotiation.
Robert Carlin, a North Korea expert at the Center for International Security and Cooperation at Stanford University in California, said North Korea's longer-range missiles may not be ready to be used for three to four years, and its nuclear program is a "low-level threat" at this point.
"We're going to get out of this particular crisis, it seems to me, without anything really blowing up," Carlin said. "But down the road, things are going to get more serious."
"What we should be looking at, really, is the decisions and the policies and the approach that we're going to have to take over the next four or five years to deal with these things," he added. "Because for the last five years, we really didn't do a very good job of doing that."
This article was written by CNN's Jethro Mullen. CNN's Jethro Mullen reported and wrote from Hong Kong. CNN's Kyung Lah, Judy Kwon and K.J. Kwon in Seoul; Tim Schwarz in Hong Kong; Barbara Starr and Elise Labott in Washington; and Matt Smith in Atlanta contributed to this report. TM & © 2012 Cable News Network, Inc., a Time Warner Company. All rights reserved.