(Bloomberg) -- The US warned North Korea of a strong punishment if it conducts a nuclear test, as Washington and a United Nations’ watchdog agency have said signs indicate Pyongyang could soon set off its first atomic device since 2017.

“There would be a swift and forceful response to such a test,” Deputy Secretary of State Wendy Sherman said in Seoul in a Tuesday meeting with her South Korean counterpart.

“Any nuclear tests would be in complete violation of UN Security Council resolutions,” she said, adding it “would be very destabilizing to the world security.” Sherman didn’t elaborate on specific measures, but suggested the US could work with its allies to levy punishments.

There’s almost no chance Russia or China, which have veto power at the UN Security council, would support any new sanctions against North Korea, as they did in 2017 following a series of weapons tests that prompted then President Donald Trump to warn of “fire and fury.” The two countries in late May vetoed a council resolution drafted by the US to ratchet up sanctions on North Korea for its ballistic missile tests this year.

The UN’s International Atomic Energy Agency said it has observed indications North Korea “may be preparing for a nuclear test.” IAEA Director General Rafael Grossi said at a meeting Monday of the agency’s Board of Governors such a test “would be a cause for serious concern.”

A test of a nuclear device would be North Korea’s seventh and the fifth under leader Kim Jong Un. His regime last tested a nuclear weapon in September 2017, which was its most powerful atomic bomb by far with an estimated yield of between 120-250 kilotons.

Any display of the weapons in Kim’s nuclear arsenal would serve as a reminder of the pressing security problems posed by Pyongyang that have simmered as US President Joe Biden’s administration has been focused on Russia’s invasion of Ukraine.

Satellite imagery indicated workers have been digging a new passageway at the Punggye-ri site, where North Korea has conducted all of its previous nuclear tests, specialist service NK News and others reported.

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