Recent satellite images of North Korea's nuclear and rocket test sites do not point to any imminent tests, a US think-tank said Friday, despite last week's threat by the communist state to flex its nuclear muscle.

The images indicate that excavation continues on a new tunnel at the remote Punggye-ri test site in the northeast, but there is little or no activity at other key parts of the facility, said the closely-followed 38 North website of the Johns Hopkins University's US-Korea Institute.

"In short, there are no indicators that a new test will be conducted in the next few months," the website said.

North Korea last week threatened to demonstrate its nuclear deterrence in a move that analysts say could indicate the regime is preparing to carry out a fourth atomic test amid long-stalled disarmament talks.

The powerful National Defence Commission (NDC), chaired by leader Kim Jong-Un, said the North would continue efforts "to bolster up its nuclear deterrence for self-defence".

"And additional measures will be taken to demonstrate its might, one after another, as long as the US nuclear threat and blackmail persist as now," the NDC said in a statement carried by Pyongyang state media.

The purpose of the ongoing excavation at Punggye-ri is unclear, 38 North said last month, adding that Pyongyang is unlikely to use this tunnel for its next test since two other tunnels at the site already appear complete.

Construction continues at the gantry and launch pad of the Sohae Satellite Launching Station on the western coast, where two launches of the long-range Unha rocket were conducted in 2012, the think-tank said, with the work unlikely to be completed for several months.

There is also no apparent test-related activity at the North's other long-range rocket launching site at Tonghae on the northeastern coast, which was last used for testing in 2009.

However, a large amount of building material has arrived, indicating construction will soon resume on Tonghae's new assembly building, 38 North said.

"While there are no signs of impending tests, activities at the three facilities indicate that Pyongyang is increasing its ability to conduct future tests," the website said.

North Korea staged its third -- and most powerful -- nuclear test in February last year after two tests in 2006 and 2009.

The reclusive communist state and its main ally China want a resumption of six-party talks on the North's nuclear weapons programme, but Washington and Seoul both insist that Pyongyang must first demonstrate some tangible commitment to abandoning nuclear weapons.