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Satellite images suggest “significant” activity at North Korea’s mountainous nuclear test site, in an area where little movement has been observed in several months, according to a U.S.-based watchdog.

In its latest analysis of satellite imagery from Sept. 8 to Nov. 1, 38 North reported Monday “significant movement of equipment, mining carts” at a tunnel in an area at the Punggye-ri test site.

Kim Jong Un and his regime conducted the North’s sixth nuclear test on Sept. 3 from an area the watchdog dubbed the “North Portal” of the mountain.

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“While it is not possible to determine the exact purpose of these activities from imagery alone, they could be associated with new nuclear test preparations at the West Portal, further maintenance on the West Portal in general and/or the abandonment of the North Portal,” the group noted in a blog post. “At the North Portal, where the last five of six nuclear tests were conducted, two temporary structures near the portal’s entrance that were likely associated with the September test have been removed, and no vehicles, mining equipment or materials have been observed in this area since the test.”

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Last week, South Korean officials said they believe the North may be planning another nuclear test or missile launch after detecting “active movement” around the North’s missile research facility.

South Korea’s National Intelligence Service (NIS) suggested to lawmakers on Nov. 2 that North Korea could be working towards replacing spent fuel rods at some point this year and the agency had already detected activity around the missile research facility in Pyongyang, Yonhap news agency reported.

“There is a possibility of a new missile launch given the active movement of vehicles around the missile research institute in Pyongyang. The North will constantly push for further nuclear tests going forward, and the miniaturization and diversification of warheads,” the NIS was quoted as saying.

The North hasn’t launched a missile since lobbing one over Japan on Sept. 15.

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According to the South Korean news agency, NIS said a tunnel at North Korea’s mountainous nuclear site was primed for another test “at any time” but also noted the possibility of damage at the Punggye-ri test site.

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According to Japanese broadcaster TV Asahi, up to 200 people were killed when a tunnel at the North’s nuclear test site collapsed in the days following the Sept. 3 test of an apparent hydrogen bomb.

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However, 38 North’s recent analysis said the reported tunnel collapse couldn’t “be corroborated with available commercial satellite imagery.”

“Significant movement of equipment and material has been observed near the West Portal since the most recent test, providing sufficient evidence that mining personnel have been inside the West Portal,” the watchdog stated. “But while the three most recent post-test tremors could have caused some damage to the tunnel networks, no observable signs of such a tunnel collapse or intensive rescue/recovery operations are visible outside any of the portals or within any of the support areas.”

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The watchdog went on to point out the “Punggye-ri Nuclear Test Site is neither abandoned nor mothballed.”

“Significant tunnel-related operations continue at the West Portal, while the South Portal remains in a continuing state of nuclear test readiness,” 38 North stated.

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The report comes as U.S. President Donald Trump met with South Korean President Moon Jae-in in Seoul where the North’s nuclear ambitions dominated talks.

Trump said that while “we hope to God” not to have to resort to the use of full U.S. military might, he was ready to do whatever was necessary to prevent the “North Korean dictator” from threatening millions of lives.

Trump also urged North Korea to “do the right thing” and added that: “I do see some movement,” though he declined to elaborate.

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“It really makes sense for North Korea to come to the table and make a deal,” Trump told reporters at a joint news conference with Moon.

— With files from Reuters.