Recent satellite images show only "limited" activity at North Korea's nuclear test site, despite reports that Pyongyang is on the verge of conducting a fifth underground test, a US think-tank said Tuesday.

But while the images contain little to suggest a test is imminent, analysts at the US-Korea Institute at Johns Hopkins University noted that North Korea has become increasingly expert at masking its preparation work.

South Korean President Park Geun-Hye told her cabinet Monday that intelligence sources had detected signs that Pyongyang was preparing a new test, an assessment echoed the same day by her defence ministry.

North Korea is gearing up for a rare and much-hyped ruling party congress early next month, at which leader Kim Jong-Un is expected to take credit for pushing the country's nuclear weapons programme to new heights.

Numerous analysts have suggested the regime might carry out a fifth nuclear test as a display of defiance and strength just before the congress opens.

In its analysis on Tuesday, the US-Korea Institute said the most recent satellite imagery of the Punggye-ri test site showed "very limited activity", although key areas were clear of snow and being maintained.

"While there is little evidence that a test is imminent, the possibility cannot be ruled out since the North has demonstrated the ability to conduct detonations on short notice by slow rolling preparations, masking significant indicators from satellite view," it said.

North Korea has conducted four nuclear tests. The most recent was on January 6, to which the UN Security Council responded by imposing its strongest sanctions to date over the North's nuclear weapons programme.