New kind of nuclear test still an option: N Korea

North Korea says it may still go ahead and test a new kind of nuclear device following US President Barack Obama's visit to Seoul, but is keeping analysts guessing as to when that test might take place.

Many experts, and the South Korean government, had suspected the North would conduct its fourth nuclear test during Obama's visit.

But the US president has come and gone and the North is now sending signals that could be taken to mean it is ready to test at any time, or may hold off indefinitely.

So will they, or won't they? Analysts remain divided over whether the activity means the North is ready to detonate, or just going through the motions.

It is notoriously difficult to divine the intentions of North Korea's isolated regime, particularly on nuclear tests when most crucial activity happens underground. Commercial satellite imagery is relatively infrequent and provides only a snapshot of what's happening.

Right now, the images suggest something is indeed underfoot. According to the newest images that have been released to the public, activity continues near tunnel entrances at the northeastern mountain testing site of Punggye-ri, where North Korea has conducted three nuclear tests since 2006, the latest in February 2013.

Experts believe the country has developed a handful of crude nuclear devices and is working toward building a warhead small enough to mount on a long-range missile, although most experts say that goal may take years to achieve.

Among other signs of preparation, the imagery by a commercial Digital Globe satellite analysed and released yesterday by the Institute for Science and International Security, showed the presence of three black vehicles and a lighter-colored vehicle or truck on the small road leading directly to the South Portal tunnel entrance.

It said that may suggest a VIP visited the site on April 29. "All these activities are consistent with the view that a test or tests will occur soon, as has been reported in South Korean media," it said.

"However, the exact timing of a test or tests is difficult to construe from the new activity.

It is also entirely possible that all the activity is a ruse. The North has been known to do that, too.