Picture used for illustrative purposes only.

Preparation work at the Punggye-ri Nuclear Test Facility's Tunnel No. 3 was apparently complete and ready for a possible nuclear test, the Center for Strategic and International Studies said in a report, citing commercial satellite imagery.

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North Korea conducted six underground nuclear tests at the site from 2006 to 2017.

The research group said that for the first time, analysts spotted new construction activity at the facility's Tunnel No. 4, "strongly suggesting an effort to re-enable it for potential future testing".

North Korean leader Kim Jong Un attends a meeting. File photo

Outside Tunnel No. 3, images showed a retaining wall and some minor landscaping with small trees or bushes, likely in anticipation of a visit by senior officials, it said.

The two tunnels were never previously used for nuclear tests and their entrances were demolished in 2018, when North Korea declared a self-imposed moratorium on testing nuclear weapons and its intercontinental ballistic missiles (ICBMs).

Leader Kim Jong Un has said he is no longer bound by that moratorium because of a lack of reciprocal steps by the United States during denuclearisation talks, and North Korea resumed testing ICBMs this year.

South Korean officials said this week that North Korea was poised to conduct a nuclear test "at any time" and that the timing would be decided by Kim.

South Korea's defence ministry spokesman, when asked about the report, said it was closely monitoring developments on North Korea's nuclear activity together with U.S. intelligence authorities but declined to make any further comment.

Reuters