North Korean leader Kim Jong Un has expressed his willingness to have foreign experts inspect the facilities, as part of on-off negotiations on denuclearisation, but there is no indication as to whether invitations have been sent.

At a parliamentary audit on Wednesday, the South's National Intelligence Service (NIS) said it was "closely monitoring" the Punggye-ri nuclear test facility and the Tongchang-ri missile launching ground for possible inspections.

"Signs have been detected that North Korea is doing some preparations and intelligence-related activity over a possible visit by outside experts," a lawmaker who was briefed by the NIS was cited as saying.

At a summit with South Korean President Moon Jae-in in Pyongyang last month, Kim pledged to close the Tongchang-ri site and allow international experts to observe its dismantlement.

North Korea took apart the Punggye-ri site in the country's northeast in May but has yet to allow international observers in to verify its claims.

US Secretary of State Mike Pompeo said that Kim had told him he was "ready" to allow foreign inspectors to verify the work at Punggye-ri, following a meeting in Pyongyang earlier this month.

The inspectors will be allowed in as soon as the two sides agree on "logistics", Pompeo said.

The facility, buried inside a mountain near the border with China, was the staging ground for all six of the North's nuclear tests.

US President Donald Trump and Kim met in Singapore in June for the first-ever summit between the two countries, resulting in what critics say was only a vague commitment by the North Korean leader towards denuclearisation.

Washington and Pyongyang have sparred over the exact terms of the agreement, with the US pushing to maintain sanctions and pressure against the North until its "final, fully verified denuclearisation".

Last month, the North's foreign minister told the United Nations there was "no way" his country would disarm first as long as tough US sanctions remain against his country.