New satellite imagery confirms continued activity at North Korea's main nuclear test site that is consistent with preparations for an atomic detonation, analysts say.

Images taken on Tuesday show particular activity near the entrances to the two completed tunnels in the South Portal area of the Punggye-ri test site, the Institute for Science and International Security (ISIS) said.

What appeared to be three black vehicles and a lighter-coloured truck could be seen on the road leading to one of the tunnels.

"All these activities are consistent with the view that a test will occur soon," the Washington-based ISIS said.

"However, the timing of a test is difficult to construe from the new activity," it added.

Concern has been building over the past week that the North is preparing to conduct a fourth test in the face of stern international opposition.

There was initial speculation that Pyongyang might time the test to coincide with an Asia tour by US President Barack Obama which included a trip to South Korea and wrapped up in the Philippines on Tuesday.

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

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.

While in Seoul last week, Obama angered the North by demanding that it abandon its nuclear weapons program, and by threatening tougher sanctions if it went ahead with another test.