North Korea fired three missiles, including one thought to be its largest intercontinental ballistic missile on Wednesday, after U.S. President Joe Biden ended an Asia trip in which he agreed to new measures to deter the nuclear-armed state.

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."

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 U.S. and the South's Hyunmoo-2 SRBM, both militaries said.

In a phone call with U.S. Secretary of Defense Lloyd Austin, South Korea's defence chief called for deploying American strategic assets and both sides agreed to reinforce U.S. 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.

Washington and Seoul officials had recently warned that North Korea appeared ready for another weapons test, possibly during Biden's visit, which was his first trip to Asia as president and included a summit with South Korean President Yoon Suk-yeol in Seoul.

The JCS said the three missiles were fired on Wednesday in less than an hour from the Sunan area of the North's capital, Pyongyang, where its international airport has become a hub of missile tests.

The first missile appeared to be the North's largest ICBM, the Hwasong-17, while a second unspecified missile appears to have failed mid-flight, said South Korea's deputy national security advisor, Kim Tae-hyo. The third missile was a short-range ballistic missile (SRBM), a test seen as aimed at improving its nuclear delivery capability, he said.

A military source in Seoul told Reuters that the second and third missile were believed to be KN-23 SRBMs, which were first tested in 2019 and which experts said designed to evade missile defences by flying on a lower, "depressed" trajectory.

"We think it had political intentions, to test our new administration's security readiness ... and send a strategic message to South Korea and the United States after President Biden left," Kim told a briefing.

MESSAGE TO ALLIES

In Seoul over the weekend, Biden and Yoon agreed to hold bigger military drills and deploy more U.S. strategic assets if necessary to deter North Korea's intensifying weapons tests.

They also offered to send COVID-19 vaccines to North Korea, which is battling its first confirmed outbreak, and urged Pyongyang to return to diplomacy.

But there has been no response from Pyongyang to the diplomatic overtures or offers of aid, Biden said.

Yoon, who took office on May 10, convened his first meeting of the national security council, which strongly condemned the latest launch as a "grave provocation", noting that Biden was on a return trip.

"North Korea's continued provocations will only result in even stronger, faster South Korea-U.S. deterrence, and bring deeper isolation upon itself," Yoon's government said in a separate statement.

A White House official said that Biden, who departed Japan on Tuesday evening, had been briefed on the launches. A State Department spokesperson issued a statement urging the North to "refrain from further provocations and engage in sustained and substantive dialogue."

South Korean Foreign Minister Park Jin and U.S. Secretary of State Antony Blinken agreed in a phone call to step up efforts to reinforce extended deterrence and facilitate a new U.N. sanctions resolution, Seoul's ministry said.

SHOW OF FORCE

South Korea had described Pyongyang's Hwasong-17 test in March as a failure, and the launch ended its self-imposed 2017 moratorium on long-range missile and nuclear testing amid stalled denuclearisation talks with Washington.

In Wednesday's test, the suspected ICBM flew 360 km (223.7 miles) to a maximum altitude of 540 km, while the SRBM flew 760 km to a maximum altitude of 60 km, the JCS said.

Japan reported at least two launches, one of which flew about 300 km and reached a maximum altitude of 550 km, and the other to the distance of about 750 km (465 miles) and a maximum altitude of 50 km, Japan's defence minister said.

Japanese chief cabinet secretary Hirokazu Matsuno said the North could take more provocative actions, including a nuclear test.

The U.S. military's Indo-Pacific Command said it was aware of "multiple" launches. They highlighted the "destabilising impact of the DPRK's illicit weapons programme" but did not pose an immediate threat.

The waning hours of Biden's visit to the region also saw Russian and Chinese bombers flying joint patrols near Japanese and South Korea air defence zones on Tuesday in a pointed farewell.