The report came hours after North Korea's military warned that it has been authorized to attack the U.S. using "smaller, lighter and diversified" nuclear weapons. It was the North's latest war cry against America in recent weeks. The reference to smaller weapons could be a claim that North Korea has improved its nuclear technology, or a bluff.
The North is not believed to have mastered the technology needed to miniaturize nuclear bombs enough to mount them on long-range missiles.
South Korean Defense Minister Kim Kwan-jin said he did not know the reasons behind the North's missile movement, and that it "could be for testing or drills."
He dismissed reports in Japanese media that the missile could be a KN-08, which is believed to be a long-range missile that if operable could hit the United States.
Kim told lawmakers at a parliamentary committee meeting that the missile has "considerable range" but not enough to hit the U.S. mainland.
The range he described could refer to a mobile North Korean missile known as the Musudan, believed to have a range of 3,000 kilometers (1,800 miles). That would make Japan and South Korea potential targets - along with U.S. bases in both countries - but there are doubts about the missile's accuracy.
The Pentagon announced that it will hasten the deployment of a missile defense system to the U.S. Pacific territory of Guam to strengthen regional protection against a possible attack.
Experts say North Korea has not demonstrated that it has accurate long-range missiles. Some suspect that an apparent long-range missile unveiled by the North at a parade last year was actually a mockup.
"From what we know of its existing inventory, North Korea has short- and medium-range missiles that could complicate a situation on the Korean Peninsula (and perhaps reach Japan), but we have not seen any evidence that it has long-range missiles that could strike the continental U.S., Guam or Hawaii," James Hardy, Asia Pacific editor of IHS Jane's Defence Weekly, wrote in a recent analysis.
Kim, the South Korean defense minister, said that if North Korea were preparing for a full-scale conflict, there would be signs such as the mobilization of a number of units, including supply and rear troops, but South Korean military officials have found no such preparations.
"(North Korea's recent threats) are rhetorical threats. I believe the odds of a full-scale provocation are small," he said. But he added that North Korea might mount a small-scale provocation such as its 2010 shelling of a South Korean island, an attack that killed four people.
North Korea has been railing against joint U.S.-South Korean military exercises that are taking place in South Korea and has expressed anger over tightened U.N. sanctions for its February nuclear test. Many of the threats come in the middle of the night in Asia - daytime for the U.S. audience.
Analysts say the threats are probably efforts to provoke softer policies from South Korea, to win diplomatic talks with Washington and to solidify the image of young North Korean leader Kim Jong Un.
At times, North Korea has gone beyond rhetoric.
On Tuesday, it announced it would restart a plutonium reactor it had shut down in 2007. A U.S. research institute said Wednesday that satellite imagery shows that construction needed for the restart has already begun.
For a second day Thursday, North Korean border authorities denied entry to South Koreans who manage jointly run factories in the North Korean city of Kaesong. South Koreans already at the plant were being allowed to return home.
South Korea has prepared a military contingency plan should North Korea hold South Korean workers hostage in Kaesong, Defense Minister Kim said. He wouldn't elaborate.
Outraged over comments in the South about possible hostage-taking and a military response from Seoul, a North Korean government-run committee threatened to pull North Korean workers out of Kaesong as well.
North Korea's military statement Thursday, from an unnamed spokesman from the General Bureau of the Korean People's Army, said its troops had been authorized to counter U.S. "aggression" with "powerful practical military counteractions," including nuclear weapons.
It said America's "hostile policy" and "nuclear threat" against North Korea "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."
U.S. Defense Secretary Chuck Hagel said Washington is doing all it can to defuse the situation.
"Some of the actions they've taken over the last few weeks present a real and clear danger and threat" to the U.S. and its allies, Hagel said Wednesday.
South Korea's Defense Ministry said its military is ready to deal with any provocation by North Korea. "I can say we have no problem in crisis management," deputy ministry spokesman Wee Yong-sub told reporters.
The defense minister, however, was criticized by lawmakers over a North Korean defector who stole a South Korean fishing boat Wednesday night and fled back to North Korea across the western sea border.
Kim said South Korean radar had a "blind spot" in the area and South Korean troops were unaware the defector was fleeing until he almost reached the North Korean side. Lawmakers questioned his military's readiness to detect and counter enemy troops who might use similar blind spots.
This spring's annual U.S.-South Korea drills have incorporated fighter jets and nuclear-capable stealth bombers. The allies insist they are routine exercises. North Korea calls them rehearsals for a northward invasion and says it needs nuclear weapons to defend itself.
On Sunday, Kim Jong Un led a high-level meeting of party officials who declared building the economy and "nuclear armed forces" as the nation's priorities.
North Korea is believed to be working toward building an atomic bomb small enough to mount on a long-range missile. Long-range rocket launches designed to send satellites into space in 2009 and 2012 were widely considered covert tests of missile technology, and North Korea has conducted three underground nuclear tests.
"I don't believe North Korea has the capacity to attack the United States with nuclear weapons mounted on missiles, and won't for many years. Its ability to target and strike South Korea is also very limited," nuclear scientist Siegfried Hecker, a senior fellow at the Center for International Security and Cooperation at Stanford University, said this week.
In comments posted on CISAC's website, Hecker said North Korea knows a nuclear attack would be met with "a devastating nuclear response."
Hecker has estimated that North Korea has enough plutonium to make several crude nuclear bombs. Its announcement Tuesday that it would restart a plutonium reactor indicated that it intends to produce more nuclear weapons material.
The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies has analyzed recent commercial satellite imagery of the Nyongbyon nuclear facility, where the reactor was shut down in 2007 under the terms of a disarmament agreement. A cooling tower for the reactor was destroyed in 2008.
The analysis published Wednesday on the institute's website, 38 North, says that rebuilding the tower would take six months, but a March 27 photo shows building work may have started for an alternative cooling system that could take just weeks. Experts estimate it could take three months to a year to restart the plant.
Associated Press writers Matthew Pennington in Washington and Youkyung Lee in Seoul contributed to this report.