North Korea is building a new complex at its main rocket launch site, possibly for training and launches of road-mobile intercontinental ballistic missiles, a US think-tank said.

Satellite imagery from May 10 suggests the North is conducting a number of important construction projects at the Sohae Satellite Launching Station on its western coast, the US-Korea Institute at Johns Hopkins University said Tuesday.

"One working hypothesis is that the North is building a new complex to conduct future training and launches for mobile missiles such as the KN-08 intercontinental ballistic missile (ICBM)", it said on its website, 38 North.

"Moreover, that hypothesis is consistent with ongoing KN-08 engine tests being conducted (at) Sohae's rocket engine test stand, where a probable KN-08 first stage is currently seen on the stand, possibly left there after early April 2014 tests or for use in the future".

Three KN-08 rocket engine test series have been identified for the first and possibly second stages dating back to mid-2013, the institute said early this month, adding the next technically logical step would be a flight test of the entire system.

North Korea successfully put a satellite into orbit in December 2012 on a rocket -- the Unha 3 -- that Pyongyang said was designed for purely scientific missions.

The international community said the launch was a disguised ballistic missile test and the UN Security Council tightened existing sanctions as a result.

The May 10 imagery also indicates that Pyongyang's effort to upgrade the existing Sohae launch pad to handle space launch vehicles larger than the Unha-3 is continuing but work has slowed, possibly due to the greater priority placed on these new construction projects.

"As a result, North Korea will be unable to conduct SLV (space launch vehicle) tests from this site until at least mid to late summer 2014 when work should be completed", it said.

The successful 2012 satellite launch caused serious concern, but experts stressed that it lacked the re-entry technology needed to bring an ICBM down onto a target.

Full-scale models of the road-mobile KN-08 missile were given pride of place in North Korean military parades in 2012 and in July last year.

But several experts ridiculed the models, with at least one respected aerospace engineer labelling them technically preposterous and a "big hoax".

The North is developing a working ICBM as a national priority and a successful test of such a missile would take the nuclear threat posed by Pyongyang to an entirely new level.

The institute said last week that despite fears to the contrary, satellite imagery suggests North Korea is not preparing an imminent nuclear test.

Imagery dated May 9 does show high levels of activity at the Punggye-ri test site, but most of it seems to be of a mundane, routine nature that would not be consistent with an impending test, it said last week.