(CNN) U.S. intelligence satellites have spotted signs that North Korea may be preparing for an unprecedented launch of a mobile ballistic missile which could potentially hit portions of the U.S., CNN has learned.

Two U.S. officials told CNN that if the regime proceeds with a launch, the latest assessments are the most likely scenario is the launch of the so-called Musudan missile, which the U.S. believes could potentially hit Guam and perhaps Shemya Island in the outer reaches of Alaska's Aleutian chain.

However, officials are strongly saying there are two other scenarios that are possible: North Korea could launch either its Kn-08 or Kn-14 mobile ballistic missiles which would have a longer range and could potentially hit the Pacific Northwest of the United States. The Kn-14 is thought to be a more precise version of the Kn-08, and it is believed the regime showed it for the first time at a military parade in 2015, officials say.

But U.S. officials also caution the regime could still decide to do nothing. North Korea is well aware U.S. spy satellites keep constant watch and the moves could be part of a deception effort to persuade the U.S. the North is about to take action.

If the North Koreans proceed, it would be the first time North Korea has launched a longer range ballistic missile from a mobile launcher and the first time any of these three missiles have flown.

South Korea's military is closely monitoring for the possibility of a fifth nuclear test by North Korea, according to a South Korean military official.

The official told CNN the military has been monitoring it since March 15, when North Korean leader Kim Jong Un gave orders to test a nuclear warhead and a ballistic rocket capable of launching a nuclear warhead. The official said this includes the Musadan missile, the Kn-08 and others that are said to be capable of reaching parts of the U.S.

Photos: North Korea launches rocket Photos: North Korea launches rocket The Kwangmyongsong carrier rocket blasted off from the Sohae launch facility at 9 a.m Sunday (7:30 p.m. ET Saturday), entering orbit nine minutes and 46 seconds after liftoff, North Korea's state news agency KCNA reported. Hide Caption 1 of 8 Photos: North Korea launches rocket A state TV newsreader said the operation had been personally ordered and directed by North Korean leader Kim Jong Un. Hide Caption 2 of 8 Photos: North Korea launches rocket State media reported that more satellite launches were planned. Hide Caption 3 of 8 Photos: North Korea launches rocket U.N. Secretary-General Ban Ki-moon called the launch "deeply deplorable" and in violation of Security Council resolutions "despite the united plea of the international community against such an act. Hide Caption 4 of 8 Photos: North Korea launches rocket North Korean leader Kim Jong Un looked on as the rocket blasted off. Hide Caption 5 of 8 Photos: North Korea launches rocket State media captures a moment on the rocket at lift off. Hide Caption 6 of 8 Photos: North Korea launches rocket The United States, South Korea, Japan, China, Russia, Britain and France, as well as the European Union and NATO, were swift to condemn the action. Hide Caption 7 of 8 Photos: North Korea launches rocket An official photograph shows the control room. Hide Caption 8 of 8

It's not clear if the missiles would work and how precise the guidance systems might be, but the estimated ranges of these missiles cause growing concern. Shemya Island, for example, houses an early warning radar installation that monitors space and missile activities.

If there is a launch, it's not clear if the missiles would carry any kind of simulated warhead.

But if the North were, for the first time, to launch a mobile missile with these types of ranges, it would be a significant military advance and a change in the North Korean calculus for the U.S., military officials say. In a conflict, mobile launchers can quickly shoot and move to a new position making it very difficult for satellites or spy planes to track them. It would also be a violation of U.N. resolutions banning North Korea from ballistic missile tests

North Korea already has twice successfully launched a three-stage ballistic missile from a stationary launch pad, both times carrying a rudimentary satellite on the front end that was sent into orbit. Because it was a launch from a fixed site, satellites were able to watch for days as preparations were made.

The rocket launch was part of a series of provocative actions by North Korea, including a January 6 nuclear test that prompted international condemnation.

Now the question for both stationary and mobile missiles is whether North Korea has miniaturized a nuclear warhead and mastered the technology of a re-entry vehicle that would give it the full capability to launch a nuclear attack.

As CNN reported in March, some U.S. intelligence analysts now believe that North Korea "probably" possesses a miniaturized nuclear warhead , several U.S. officials told CNN.

The assessment has yet to become a formal consensus view of the U.S. government. But it reveals just how far along many in the U.S. believe the reclusive country has come to gaining a nuclear-tipped ballistic missile that could potentially strike the U.S.

As North Korean leader Kim Jong Un's public rhetoric has escalated in recent weeks, concern has grown inside intelligence circles that he has made progress on several fronts.

"He is determined to prove his doubters wrong," one U.S. official told CNN, even as uncertainty remains about how much progress he has actually made in his quest for nuclear missiles.

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Recent photos showing Kim standing next to what the North Koreans claim is a miniaturized nuclear device are still being scrutinized by U.S. analysts for any indication of progress, officials said, declining to provide additional specifics.

U.S. officials who endorse the notion that Kim probably has a nuclear warhead still note that they don't know if the device would actually work. The North Koreans believe it would.

U.S. commanders have said they assume for war planning purposes that North Korea has a functional warhead but have stopped short of outright declaring it exists.

"It's the prudent decision on my part to assume that he has the capability to miniaturize a nuclear weapon and put it on an ICBM," Adm. William Gortney, head of U.S. Northern Command, recently told Congress.

But the Pentagon has taken pains to downplay the possibility. Pentagon press secretary Peter Cook acknowledged that "the commanders who are responsible for these activities" are "doing the prudent, appropriate, proper" thing by assuming the North Koreans possess this capability.