SEOUL (AFP) - New satellite imagery indicates North Korea has been testing the engine for an inter-continental ballistic missile (ICBM), a United States think-tank said on Friday, amid concerns the North is also preparing a nuclear test.

The US-Korea Institute at Johns Hopkins University said images of the North's main rocket launch site suggested one "and maybe more" tests of what is probably the first stage of a KN-08 road-mobile ICBM.

With this latest activity, 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 on its closely followed website, 38 North.

"As this effort progresses, the next technically logical step in the missile's development would be a flight test of the entire system," it said.

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 United Nations Security Council tightened existing sanctions as a result.

The Unha-3 stood 30m high, and the 38 North post said the satellite images showed the gantry at the Sohae launch site was being modified to take larger rockets of up to 50 metres in height.

"Construction may not be completed until early summer, effectively preventing a launch from the facility in the meantime," it said.

The signs of engine testing come amid concerns that the North is on the verge of carrying out a fourth nuclear test in the face of stern international opposition.

Separate satellite imagery of the North's main Punggye-ri nuclear test site has shown stepped-up activity consistent with preparations for a test.

Missile delivery has often been cited as the main weakness of the North's nuclear weapons programme which, after three tests, is believed to be close to mastering the key technology of warhead miniaturisation.

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.

Models of the road-mobile KN-08 missile have been 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".

If there is disagreement over how close the North might be to a reliable, working ICBM, there is no doubt that developing one is a national priority.

And a successful test of such a missile would take the nuclear threat posed by the North to an entirely new level.