There are increasing signs that North Korea could soon test a nuclear weapon for the first time since 2017 in a bid to improve its arsenal and increase political pressure, U.S. and South Korean officials and analysts said.
Two U.S. officials told Reuters that there were indications, including activity near the Punggye-ri nuclear site, that Pyongyang may be preparing for some sort of test, though an exact timing was unclear.
A South Korean military official confirmed that they were tracking activity to restore one of the tunnels used for nuclear tests.
Pentagon spokesman John Kirby declined this week to comment, but said Washington is concerned about the possibility of new tests because they would be an opportunity for North Korea to enhance its arsenal.
"Every time you test you learn.... We know that this is a programme that they want to improve," he told a briefing on Tuesday. "And so of course, we're concerned about efforts to do that."
Analysts say that more testing could help North Korea reach its stated goals of making smaller nuclear warheads and improving their reliability.
A resumption of nuclear tests could send political shockwaves through the region. China and Russia had joined the United States and other United Nations Security Council members in sanctioning Pyongyang over its previous tests, but in the wake of last week's intercontinental ballistic missile (ICBM) flight, both Beijing and Moscow signalled opposition to any new measures and said sanctions should be eased.
Liu Xiaoming, China's envoy for Korean affairs, has called on all sides to show restraint, but said the root cause of tensions is Washington's failure to address North Korea's legitimate security concerns and to reciprocate steps Pyongyang had taken since 2018.
On Thursday State Department spokesman Ned Price said Washington remains open to talks, but that continued provocations by North Korea would incur additional responses from the international community.
A Nuclear Site Reborn
North Korea has conducted all six of its nuclear tests in deep tunnels dug under the mountains at Punngye-ri. In 2018 it used explosives to close old entrances in front of invited foreign media but not international experts, raising questions about the extent of the demolition.
That year Pyongyang declared a voluntary moratorium on testing nuclear weapons and its ICBMs. Since then, it has said it is not bound to that because of a lack of reciprocal moves by the United States and its allies. Last month, it test-fired ICBMs for the first time since 2017.
Commercial satellite imagery from Thursday shows probable new excavation at the site's South Portal, just east of a former tunnel entrance that was destroyed as part of site dismantlement efforts in 2018, 38 North, a U.S.-based programme that monitors North Korea, said in a report.
Although some South Korean media reports suggested that workers were building "shortcuts" to connect with the test tunnels as quickly as possible, it seems more likely they were trying to excavate into a stable point rather than digging through the fractured rock around the former entrance, 38 North said.
The organisation noted that some technical buildings such as the site's command and control centre were not destroyed in 2018.
Since December, satellite imagery has showed activity at the main administrative area, the Vienna-based Open Nuclear Network (ONN) said in a report this week.
Notably, the South Portal tunnel that North Korea appears be reactivating was not previously used for testing, the ONN report said. Piles of what might be logs, often used to shore up such tunnels, have also been spotted, it added.
A separate 38 North report said satellite imagery shows increased activity around North Korea's Sohae Satellite Launching Station, after leader Kim Jong Un ordered its expansion as part of a programme to launch spy satellites to monitor military moves by the United States and its allies.
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