A view of what state news agency reports is the test firing of a hypersonic missile in North Korea. File/Reuters

South Korea's deputy national security advisor, Kim Tae-hyo, said that the North also appeared to have conducted "multiple experiments" with a detonation device in preparation for its seventh nuclear test, but that the test is unlikely to occur "in the coming days but afterward."

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In response, the United States and South Korea held combined live-fire drills, including surface-to-surface missile tests involving the Army Tactical Missile System (ATACMS) of the US and the South's Hyunmoo-2 SRBM, both militaries said.

US President Joe Biden attends the Quad Leaders Summit at Kantei in Tokyo on Tuesday. AFP

In a phone call with US Secretary of Defense Lloyd Austin, South Korea's defence chief called for deploying American strategic assets and both sides agreed to reinforce US extended deterrence to counter the North's provocations, Seoul's defence ministry said.

"Our military's show of force was intended to highlight our resolve to firmly respond to any North Korean provocations, including an ICBM launch, and our overwhelming capability and readiness to conduct a surgical strike on the origin of the provocation," South Korea's Joint Chiefs of Staff (JCS) said in a statement.

North Korea has conducted a flurry of missile launches this year, from hypersonic weapons to test firing its largest intercontinental ballistic missiles (ICBMs) for the first time in nearly five years. It also appears to be preparing for what would be its first nuclear test since 2017.

Reuters