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A satellite photo of the Punggye-Ri nuclear test site in North Korea May 14, 2018. (Image: Planet Labs Inc/Handout via REUTERS) - 

South Korea's military said on Friday (Mar 11) it had detected activity at North Korea's nuclear test site to restore at least some tunnels that were demolished in 2018 when the site was closed.

North Korea has not tested a nuclear bomb or its longest-range intercontinental ballistic missiles since 2017 but has said it could resume such testing with denuclearisation talks with the United States stalled.

Amid a flurry of now-moribund diplomacy in 2018, North Korea used explosives to demolish the entrances to some underground tunnels at Punggye-ri, its only known nuclear test site.

"Activity to restore part of the tunnels at North Korea’s Punggye-ri nuclear test site that were destroyed on May 24, 2018, has been detected," South Korea's military said in a statement, without elaborating on the type of activity.

South Korea is closely cooperating with its US allies to monitor the activities, the statement added.

The report came after the National Security Council said on Sunday it was paying particularly close attention to Punggye-ri and the main nuclear reactor site at Yongbyon.

Images captured by commercial satellite a week ago showed very early signs of activity at the site, including the construction of a new building, repair of another building, and what was possibly some lumber and sawdust, specialists at the California-based James Martin Center for Nonproliferation Studies (CNS) said in a report released on Tuesday.

International monitors have also reported the nuclear reactor facility at Yongbyon appears to be in full swing, potentially creating fuel for nuclear weapons.

North Korea recently used what would be its largest-ever intercontinental ballistic missile system in two secretive launches, likely paving the way for a resumption of long-range tests, US and South Korean officials said on Friday//CNA