It was not immediately clear whether the missile was fired from a submarine or from a submersible test barge, as in most previous test.

“Our military is closely monitoring the situation and maintaining readiness posture in close cooperation with the United States, to prepare for possible additional launches,” the Joint Chiefs of Staff said in a statement.

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The US military’s Indo-Pacific Command condemned the launch as destabilising but assessed that it did not pose an immediate threat to the United States or its allies. The missile flew about 430-450km to a maximum altitude of 60km, a military source said.

North Korea’s last SLBM test was in October 2019, when a Pukguksong-3 missile was fired from an underwater platform, flying 450km to a maximum altitude of 910km.

The North displayed new Pukguksong-4 and Pukguksong-5 SLBMs during its military parades in October and January, respectively, and a previously unseen, smaller missile was spotted at a defence fair last week in Pyongyang.

Professor Park Won-gon, from Ewha Womans University’s department of North Korean studies, said that the recent missile tests were in accordance with Pyongyang’s five-year plan to build up its strategic weapons capability.

“The North, while insisting on the US dropping its ‘hostile policy’ and ’double standards’, has been launching missiles as a pressure tactic,” Park said.

“The North is likely to conduct such tests more often in the coming months as there is not much time left for it to do so before the start of the Beijing Winter Olympics in February, for which China wants a peaceful atmosphere”.

North Korea has also been working on what would be its first operational submarine capable of launching an SLBM, according to South Korean officials. It currently has an experimental ballistic missile submarine it claims was used in a 2016 test, though some analysts say that may have used a barge.

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The Washington-based Nuclear Threat Initiative has assessed that “an operational [ballistic missile submarine] and SLBM capability could provide North Korea with additional options for nuclear launch, and a hedge against destruction of its land-based nuclear systems”.

South Korea’s national security council held an emergency meeting and expressed “deep regret” over the test, urging the North to resume talks.

Japanese Prime Minister Fumio Kishida said that two ballistic missiles had been detected, and that it was “regrettable” that North Korea had conducted a string of missile tests in recent weeks. There was no immediate explanation for the conflicting number of missiles detected.

Kishida cancelled scheduled campaign appearances in northern Japan, and the deputy chief cabinet secretary told reporters that the prime minister was planning to return to Tokyo to deal with the missile situation.

The launch came as the intelligence chiefs of the United States, South Korea, and Japan were set to meet in Seoul to discuss the stand-off with North Korea, amid other issues, Yonhap reported, citing a government source.

The missiles tested recently by North Korea appear aimed at matching or surpassing South Korea’s quietly expanding arsenal, analysts have said.

Last month South Korea successfully tested an SLBM, becoming the first country without nuclear weapons to develop such a system. North Korea test fired a missile launched from a train on the same day.

This month the two Koreas held duelling defence exhibitions aimed at showcasing their latest weaponry amid a spiralling arms race.

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As news of Tuesday’s missile launch broke, the opening ceremonies of the International Aerospace and Defence Exhibition (ADEX) took place. It is set to be South Korea’s largest defence expo ever, organisers said, with displays of next-generation fighter aircraft, attack helicopters, drones, and other advanced weapons, as well as space rockets and civilian aerospace designs.

At least 440 companies from 28 countries are showcasing 79 types of aircraft and 68 types of ground equipment. Around 300 government, military and defence acquisition officials from 45 countries were expected to attend the event, which lasts through Saturday. About 170 US troops were on hand to display American military aircraft, the US Air Force said in a statement.

“Despite the Covid-19 situation this year, export marketing is expected with products developed by domestic companies at the exhibition hall, which has been expanded from last year,” ADEX official Lee Jong-ho told reporters on Monday.

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Last week Korea Aerospace Industries officials declined to comment on potential export deals for its KF-21 fighter jet and Light Attack Helicopters, but said both programmes were on track.

Korean Air said it was displaying drones with vertical lift-off and landing capabilities, while a “space hub” at the show displays components of South Korea’s space programme, expected to get a boost with the planned launch of its first space launch vehicle on Thursday.

South Korea is also preparing to test fire its first home-grown space launch vehicle on Thursday.

Though analysts say the South Korean rocket has few potential applications as a weapon, such tests are unlikely to be welcomed in North Korea, which has complained of a double standard in which its own space programme is criticised overseas as a front for military missile development.