A North Korean nuclear test within days "cannot be ruled out" analysts said Saturday, after new satellite imagery showed heightened activity at the test site.

The report by the Institute for Science and International Security chimes with other findings and suggests Pyongyang is moving towards an underground detonation.

Pictures taken on Friday -- the first day of U.S. President Barack Obama's visit to neighbouring South Korea -- show an increase in movement near one entrance to a tunnel.

"The images show in particular activity at the South Portal of the site, a possible site for North Korea's next nuclear test, and in the main support area," ISIS said in a release.

Analysis of satellite images taken on Wednesday and released by the US-Korea Institute at Johns Hopkins University showed increased activity at the Punggye-ri test site.

This was "probably related to preparations for a detonation," the institute said on its closely followed 38 North website.

ISIS said higher resolution imagery, taken on Friday confirmed that analysis and suggested the preparations were continuing.

"On April 23, several containers were located in front of one of the South Portal's tunnel entrances," ISIS said.

"The higher resolution April 25 imagery shows more clearly what appears to be this collection of boxes or containers near this tunnel entrance."

North Korea has conducted three nuclear tests in 2006, 2009 and 2013, despite huge international pressure to halt its program, and its activity is being keenly watched.

However, the regime is notoriously unpredictable and observers warn that it is possible the preparations are a feint for the benefit of the satellites Pyongyang knows are watching.

The North has a long record of making threats in an effort to extract aid or concessions from the international community.

But activity at the site is being watched particularly carefully because of Obama's presence in the region amid speculation Pyongyang would like to thumb its nose at the US.

"Test site preparations do not necessarily mean that a test will occur in the next few days. North Korea has made preparations before and not tested," ISIS cautioned.

Politics would certainly play a role in the North's calculations, and it would be weighing the possible impact of detonation while Obama was in Asia.

"It may guarantee a harsher political response from the President," said ISIS.

"Nonetheless, determining North Korea's plans and schedules is always fraught with uncertainty. Thus, a test in the next several days cannot be ruled out by any means."

Speaking earlier Saturday to US service personnel stationed in South Korea, Obama blasted North Korea as a "pariah state that would rather starve its people than feed their hopes and dreams".

"North Korea's continued pursuit of nuclear weapons is a path that leads only to more isolation," he said. (AFP)