Recent satellite images show no imminent signs of a fifth North Korean nuclear test, following the conclusion of a party congress that many thought Pyongyang would mark with an atomic detonation, a US think tank said Wednesday.

The pictures, dated May 8, show low-level activity at the underground test site at Punggye-ri in the country's northeast, and vehicles previously observed at the site's command centre were "no longer present", according to an analysis by the US-Korea Institute at Johns Hopkins University.

Earlier satellite images had shown closely parked vehicles at the command centre -- usually only seen during final test preparations.

North Korea wrapped up its first ruling party congress in 36 years on Monday.

South Korean officials and North Korea watchers elsewhere had warned of a high possibility that Pyongyang might -- as a show of strength -- carry out a test in the run-up to the conclave, or even during the event itself.

"That gathering is now ended and there are no apparent signs that a detonation will occur in the near future," said the analysis.

But the low-level activity across the complex suggests it "remains capable of supporting additional tests once a decision to move forward is made in Pyongyang", it said.

North Korea has conducted four nuclear tests. The most recent was on January 6, with Pyongyang claiming it marked the country's first successful test of a powerful hydrogen bomb.

The UN Security Council responded by imposing its strongest sanctions to date over the North's nuclear weapons programme.

At the ruling party congress, North Korean leader Kim Jong-Un praised the "magnificent and exhilarating sound" of the January test, and delegates adopted Kim's report calling for an improved and expanded nuclear arsenal.