North Korea has fired a projectile towards the sea off its east coast, South Korea's military said, with Japan claiming it could have been a ballistic missile.
It comes as the US urged the UN Security Council to vote on further sanctions over North Korea's missile launches.
The country's 14th major weapons test this year took place days after North Korea vowed to speed up development of its nuclear arsenal.
It also took place days before South Korea's newly elected President Yoon Suk-yeol takes office on May 10.
Details of the projectile, including its flight range and altitude, were not immediately available. Japan's Coast Guard said it could be a ballistic missile.
Last week, North Korean leader Kim Jong Un pledged to speed up the development of his country's nuclear arsenal while overseeing a huge military parade as denuclearisation talks with the United States remain stalled.
The US would like the UN Security Council to vote this month to further sanction North Korea over its renewed ballistic missile launches, US Ambassador to the UN Linda Thomas-Greenfield said on Tuesday, Reuters reported.
The US circulated an initial draft resolution to the 15-member council last month that proposed banning tobacco and halving oil exports to North Korea, and blacklisting the Lazarus hacking group.
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However, Russia and China have already signalled opposition to boosting sanctions in response to Pyongyang's March launch of an intercontinental ballistic missile — its first since 2017. A Security Council resolution needs nine "yes" votes to pass, without a veto by Russia, China, France, Britain or the US.
"It is our plan to move forward with that resolution during this month," Ms Thomas-Greenfield told reporters when asked if she would put it to a vote. The US is president of the Security Council for May.
"We're very concerned about the situation," she said. "It is our hope that we can keep the council unified in condemning those actions by the DPRK (North Korea)."
North Korea has been subjected to UN sanctions since 2006, which the Security Council has steadily stepped up over the years in a bid to cut off funding for Pyongyang's nuclear weapons and ballistic missile programmes.
But the Asian state has successfully worked to evade some sanctions, according to independent monitors who reported in February that North Korean cyberattacks on cryptocurrency exchanges were earning Pyongyang hundreds of millions of dollars.