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 (ISIS) chimes with other findings and suggests Pyongyang is moving towards an underground detonation.

Pictures taken on Friday -- the first day of US 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 programme, 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.

N. Korea 'probably' preparing nuclear test: US think-tank
Seoul (AFP) April 25, 2014 - New satellite imagery suggests North Korea is preparing to conduct its fourth nuclear test, a US think-tank said Friday just hours before President Barack Obama arrives in South Korea for a visit.

The analysis by the US-Korea Institute at Johns Hopkins University echoed recent warnings from South Korea that the North might be planning a test to coincide with Obama's two-day visit.

The satellite images, taken just two days ago, showed additional activity at the Punggye-ri test site that is "probably related to preparations for a detonation," the institute said on its closely followed 38 North website.

Analysis suggested increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels.

Also visible were probable command and control vehicles intended to provide secure communications between the test site and other facilities.

North Korea has conducted three nuclear tests in 2006, 2009 and 2013.

The 38 North post noted that preparations for the test in February last year had peaked two or three days before detonation.

The withdrawal of all equipment, vehicles and personnel had occurred immediately before the blast.

"Whether North Korea will follow the same timeline in 2014 remains unclear," it said.

The nuclear threat from Pyongyang will top Obama's agenda in South Korea, a key US regional ally which hosts a permanent deployment of 28,500 US troops.

"North Korea has engaged in provocative actions for the last several decades," Obama said in Tokyo on Thursday.

-- Obama slams 'irresponsible' N. Korea --

"It's been an irresponsible actor on the international stage for the last several decades," he added.

Any nuclear test would overwhelm the narrative of Obama's regional tour, which is designed to reinvigorate his rebalancing of US strategy towards the region.

Earlier this week, Pyongyang slammed Obama's trip as a "dangerous" move that would escalate military tension and bring the "dark clouds of a nuclear arms race" over the Korean peninsula.

South Korea's defence ministry first suggested Tuesday that stepped-up activity at Punggye-ri indicated a fourth test might be in the pipeline.

However, ministry spokesman Kim Min-Seok also acknowledged it could be a "deception tactic" aimed at rattling a few cages in the region during Obama's tour.

"We are thinking of possibilities that the North may stage a surprise nuclear test or just pretend to stage a nuclear test," Kim said.

In a briefing for the foreign media on Thursday, a senior government official said the North had sealed a tunnel at the Punggye-ri site.

Last year, Pyongyang restarted a plutonium reactor that it had shut down at its Yongbyon nuclear complex in 2007 under an aid-for-disarmament accord.

The Yongbyon reactor is capable of producing six kilograms (13 pounds) of plutonium a year -- enough for one nuclear bomb.

Pyongyang is currently believed to have enough plutonium for as many as six bombs, after using part of its stock for at least two of its three atomic tests to date.

The North warned at the end of March that it would not rule out a "new form" of nuclear test after the UN Security Council condemned its latest series of medium-range missile launches.

Experts saw this as a possible reference to testing a uranium-based device or a miniaturised warhead small enough to fit on a ballistic missile.