The movement of the mid-range missile was detected by both South Korean and US intelligence, the South's Yonhap news agency reported, citing military and government sources.
"It appeared that the object was a Musudan mid-range missile," it quoted one South Korean official as saying.
"We are closely monitoring whether the North moved it with a view to actual launch or just as a show of force against the US."
The Musudan missile was first unveiled at a military parade in October 2010 and is believed to have an intended range of around 3000km. However, it is not known to have been tested.
Yonhap cited intelligence sources as saying the North might launch the missile on April 15, the birth anniversary of founding leader Kim Il-sung.
The South Korean Defence Ministry declined to confirm the report, but stressed that it kept a "24-hour watch" for any potential North Korean missile launches.
"We believe there is always an open possibility for a missile launch and related measures have been prepared," ministry spokesman Wi Yong-Seop told reporters without elaborating.
The United States has said it is sending ground-based missile interceptors to Guam in response to North Korean threats to strike the Pacific island and other US targets, which Defence Secretary Chuck Hagel described as a "real and clear danger".
Tensions also continued between the two Koreas over a joint industrial park, with North Korea asking companies in the Kaesong zone for a list of all South Korean personnel who wish to leave the complex in the coming days.
Pyongyang told Seoul yesterday it was stopping the daily movement of South Koreans to Kaesong, which lies 10 kilometres inside North Korea and which is the last real surviving point of contact between the two countries.
The North said the South Koreans in the complex could leave when they wanted, but hundreds opted to stay to keep their plants running.
North Korea later also threatened to pull its 53,000 workers out of the Kaesong zone "if the South Korean puppets and conservative news media keep badmouthing (us)".
The potential crisis on the Korean peninsula comes after the North's move to restart operations at its main Yongbyon nuclear complex to produce more weapons.
Early this morning, in a move apparently timed to hit evening news bulletins in the US, the rogue state issued a statement saying its military had ratified plans for a nuclear strike on the US.
The statement, while provocative, repeats its earlier threats and contains the usual smattering of conditional clauses and bombast. However, it was issued in the name of the Korean People's Army's general staff, giving it added weight in the eyes of observers.
"We formally inform the White House and Pentagon that the ever-escalating US 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," the statement said.
Overnight, the Pentagon said it would deploy its Terminal High Altitude Area Defence (THAAD) system to Guam in the coming weeks. The THAAD system includes a truck-mounted launcher, interceptor missiles, an AN/TPY-2 tracking radar and an integrated fire control system.
"The United States remains vigilant in the face of North Korean provocations and stands ready to defend US territory, our allies, and our national interests," a Pentagon spokeswoman said.
While Pyongyang has successfully carried out test nuclear detonations and a long-range missile test, most experts think it is not yet capable of mounting a device on a ballistic missile capable of striking US bases or territory.
The US, though, has responded to the rhetoric by first deploying two missile-defence capable destroyers to the Korean peninsula, then a mobile X-band radar installation and now the THAAD system to Guam.
US Defence Secretary Chuck Hagel said overnight that Pyongyang represented a "real and clear danger" to the United States and to its allies South Korea and Japan.
"They have nuclear capacity now, they have missile delivery capacity now," Hagel said after a strategy speech at the National Defence University. "We take those threats seriously, we have to take those threats seriously."
"We are doing everything we can, working with the Chinese and others, to defuse that situation on the peninsula. I hope the North will ratchet its very dangerous rhetoric down," he said.
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