The South Korean and US militaries flew 20 fighter jets over waters off South Korea’s western coast Tuesday in a continued show of force as a senior US official warned of a forceful response if North Korea goes ahead with its first nuclear test explosion in nearly five years.

South Korea’s Joint Chiefs of Staff said the air demonstration involved 16 South Korean planes — including F-35A stealth fighters — and four US F-16 fighter jets and was aimed at demonstrating their ability to swiftly respond to North Korean provocations.

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The flight came a day after the allies fired eight surface-to-surface missiles into South Korea’s eastern waters to match a weekend missile display by North Korea, which fired the same number of weapons from multiple locations Sunday in what was likely its biggest single-day testing event.

North Korea may soon up the ante as US and South Korean officials say the country is all but ready to conduct another detonation at its nuclear testing ground in the northeastern town of Punggye-ri. Its last such test and sixth overall was in September 2017, when it claimed to have detonated a thermonuclear bomb designed for its intercontinental ballistic missiles.

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Travelling to Seoul to discuss the standoff with South Korean and Japanese allies, US Deputy Secretary of State Wendy Sherman warned of a “swift and forceful” response if the North carries out another nuclear test.

While the Biden administration has vowed to push for additional international sanctions if North Korea goes on with the nuclear test, the prospects for meaningful new punitive measures are unclear with the UN Security Council divided.

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“Any nuclear test would be in complete violation of UN Security Council resolutions. There would be a swift and forceful response to such a test,” Sherman said, following a meeting with South Korea Vice Foreign Minister Cho Hyun-dong.

“We continue to urge Pyongyang to cease its destabilising and provocative activities and choose the path of diplomacy,” she said.

Sherman and Cho are planning a trilateral meeting with Japanese Vice Foreign Minister Mori Takeo on Wednesday over the North Korean nuclear issue.

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North Korea’s launches on Sunday extended a provocative streak in weapons tests this year that also included the country’s first demonstrations of ICBMs since 2017.

Since taking power in 2011, North Korean leader Kim Jong Un has accelerated his weapons development despite limited resources. Experts say with its next test, North Korea could claim an ability to build small bombs that could be clustered on a multiwarhead ICBM or fit on short-range missiles that could reach South Korea and Japan.