North Korea fired what was believed to a ballistic missile on Tuesday, South Korea's military and the Japanese coastguard said.
The launch was detected about 7.27am from an inland area of North Korea towards the ocean off its east coast, South Korea's Joint Chiefs of Staff said.
The projectile appeared to have landed outside Japan's exclusive economic zone, the Kyodo news agency reported, quoting government sources in Tokyo.
The launches by nuclear-armed North Korea underscored Mr Kim's New Year's vow to strengthen the military against an unstable international situation amid stalled talks with South Korea and the US.
"The [South Korean] military is maintaining a readiness posture while closely monitoring related trends under close co-operation between [South Korea] and the US in preparation for additional launches," the joint chiefs said.
South Korean and US intelligence agencies are conducting detailed analysis for additional information, it said.
Japan's Prime Minister Fumio Kishida said North Korea's repeated missile launches were "very regrettable".
Last week, South Korean military officials cast doubts on the capabilities of a "hypersonic missile" North Korea claimed to have tested on Wednesday, saying it appeared to represent limited progress on Pyongyang's ballistic missiles.
Tuesday's launch came a day after the US mission to the UN, joined by France, Ireland, Japan, the UK and Albania, condemned last week's test.
"These actions increase the risk of miscalculation and escalation and pose a significant threat to regional stability," US ambassador Linda Thomas-Greenfield said on Monday.
Such tests not only improve the North's capabilities, but expand what it can offer illicit arms clients and dealers around the world, Ms Thomas-Greenfield said.
"[North Korea] makes these military investments at the expense of the well-being of the North Korean people," she said.
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North Korean leader Kim Jong-un attends the eighth conference of military educationists of the Korean People's Army at the April 25 House of Culture in Pyongyang. Korean Central News Agency / AFP -
Kim Jong-un attends a military parade marking the ruling party congress at Kim Il-sung Square in Pyongyang in January 2021. KCNA / AP -
Kim Jong-un is applauded by a military unit after what North Korea claimed was a test firing of its 'super-large' multiple rocket launcher in November 2019. KCNA / AP -
Kim Jong-un rides a horse to climb Mount Paektu in North Korea in October 2019. KCNA / AP -
Kim Jong-un and former US president Donald Trump shake hands over the military demarcation line at the border village of Panmunjom in the demilitarised zone in June 2019. KCNA / AP -
Kim Jong-un and South Korean President Moon Jae-in raise their hands after signing a joint statement in Panmunjom in April 2018. Korea Summit Press Pool / AP -
Kim Jong-un inspects a launch drill of the medium and long-range strategic ballistic rocket Hwasong-12 at an undisclosed location in September 2017. KCNA / AFP -
Kim Jong-un smiles with soldiers after inspecting the multiple-rocket launching drill of the women's sub-units under the Korean People's Army Unit 851 at an undisclosed location in North Korea, April 2014. KCNA / AFP -
Kim Jong-un talks to former NBA star Dennis Rodman as they watch an exhibition basketball game at an indoor stadium in Pyongyang in January 2014. KCNA / AP -
Kim Jong-un inspects a mass parade of the Worker-Peasant Red Guards at Kim Il-sung Square in Pyongyang to mark the 65th anniversary of national foundation day, September 2013. KCNA / AFP -
Kim Jong-un visits watch posts of KPA Unit 507 in the Kangwon province of North Korea, June 2013. KCNA / AFP -
Kim Jong-un enjoys the ride as he attends the completion ceremony of the Rungna People's Pleasure Ground in Pyongyang in July 2012. KCNA / AP -
Kim Jong-un is surrounded by members of the Korean Children's Union in Pyongyang in June 2012. KCNA / AFP -
Kim Jong-un attends a meeting of the Supreme People's Assembly in Pyongyang in April 2012. KCNA / AFP -
Kim Jong-un aims a gun on an inspection tour of the Sporting Bullet Factory in Pyongyang in February 2012. KCNA / AFP -
Kim Jong-un salutes besides the convoy carrying the body of his father and late leader Kim Jong-il at Kumsusan Memorial Palace in Pyongyang in December 2011. KCNA / AFP -
Late North Korean leader Kim Jong-il with his son Kim Jong-un on the balcony as they attend a military parade marking the 65th anniversary of the communist nation's ruling Workers' Party in Pyongyang, October 2010. Kyodo News / AP -
Kim Il-sung with his son and chosen successor, Kim Jong-il in November 1986. KCNA / AFP
UN Security Council resolutions ban all ballistic missile and nuclear tests by North Korea, and the body has imposed sanctions over the programmes.
Ms Thomas-Greenfield repeated calls for countries around the world to enforce sanctions, and for North Korea to return to talks and abandon its missiles and nuclear weapons.
"Our goal remains the complete, verifiable and irreversible de-nuclearisation of the Korean Peninsula," she said.
The US State Department did not immediately respond to a request for comment about Tuesday's launch.
North Korea has said it is open to talks, but only if the US and others drop "hostile policies" such as sanctions and military drills.
Few observers expect Mr Kim to ever fully surrender his nuclear arsenal.
North Korea says its missile tests and other military activities are for self-defence and are similar to those regularly undertaken by other nations.
