Seoul - North Korea has stepped up tunnelling at its main nuclear test site, a US think tank said, even as tensions cool on the peninsula following the resumption of long-stalled inter-Korean dialogue.

Satellite images showed increased activity at the Punggye-ri site, with mining carts and personnel frequently visible, and excavation waste piles growing, the respected 38North website said on Thursday.

"These activities underscore North Korea's continued efforts to maintain the Punggye-ri site's potential for future nuclear testing," it said.

The last five of Pyongyang's six nuclear tests have been carried out under Mount Mantap at Punggye-ri in the country's north-west, all of them at the North Tunnel.

After a series of small earthquakes in the area, 38North said last October that the site may be suffering from the geological condition "Tired Mountain Syndrome".

The syndrome refers to the effect of below-ground nuclear blasts on the surrounding rock, which is extensively fractured and becomes increasingly permeable.

The latest pictures indicated the North Tunnel was "dormant" with water draining from its entrance, but that "tunnel excavation has been stepped up at the West Portal".

Elsewhere at the site there was unusual activity with some 100-120 personnel lined up in formation in the courtyard, their purpose unknown.

The images were taken in December, shortly before North Korean leader Kim Jong-Un gave a New Year speech warning US President Donald Trump of a "nuclear button" on his desk, while also offering to hold talks with Seoul.

The olive branch led to the first official dialogue between the two Koreas in more than two years, and an agreement for Pyongyang to send a delegation to next month's Winter Olympics in the South.

The apparent rapprochement came after months of confrontation, during which Pyongyang carried out multiple missile tests and by far its biggest nuclear detonation to date.

But North Korea made no promises on its weapons programme at the talks, and lashed out when asked about denuclearisation by South Korean journalists, saying it was not up for discussion.

"The target of all our nuclear and hydrogen bombs and ICBMs and all other sophisticated weapons is the US," the North's chief delegate Ri Son-Gwon said.

Since Kim inherited power in 2011, North Korea has made rapid progress towards its goal of developing a missile that can deliver an atomic warhead to the United States, which significantly strengthens its negotiating position.