North Korea fired three unidentified projectiles into the sea on Monday, Seoul's military said, the second such weapons test by Pyongyang in a week.
"The military is monitoring for additional launches and maintaining readiness," it added.
A spokesman at Japan's defence ministry said North Korea launched what appeared to be "ballistic missile(s)", adding there had been no indication of anything coming down in Tokyo's territory or exclusive economic zone.
Analysts say the North has been continuing to refine its weapons capabilities during its long-stalled nuclear discussions with the U.S., which have been at a standstill since the collapse of the Hanoi summit between leader Kim Jong Un and President Donald Trump more than a year ago.
Three projectiles fired successfully from a single Transporter Erector Launcher (TEL) would be "a new milestone" for the North's short-range ballistic missile programme, tweeted Ankit Panda, senior fellow at the Federation of American Scientists.
Vipin Narang of the Massachusetts Institute of Technology added: "Kim continues to test, improve, and operationalize his force."
After last week's launch the North's state media said Kim had overseen a "long-range artillery" drill, carrying images of multiple launch rocket systems and several of a larger calibre rocket being fired in a forest.
South Korea said that launch appeared to be two short-range ballistic missiles -- which the nuclear-armed North is banned from testing under UN Security Council resolutions.
Personal letter
Monday's launch came days after Mr. Kim sent a personal letter to the South's President Moon Jae-in, offering "comfort" for the rapid outbreak of the new coronavirus in the country.
South Korea has one of the world's largest infection totals outside China with more than 7,300, while Pyongyang insists it has not had a single case.
That message had followed an unprecedented statement by Mr. Kim's younger sister Yo Jong, when she berated Seoul for its condemnation of Pyongyang's weapons test last week was a "truly senseless act" and "perfectly foolish".
The North carried out a series of weapons tests late last year, the last of them in November, which it often described as multiple launch rocket systems although others called them ballistic missiles.
It also conducted static engine tests, most recently in December.
Pyongyang set Washington a unilateral deadline of the end of 2019 to offer it fresh concessions on sanctions relief, and at a party meeting in late December Kim declared the North no longer considered itself bound by its moratoriums on nuclear and intercontinental ballistic missile tests.
He also threatened a demonstration of a "new strategic weapon" soon.
Pyongyang is under multiple sets of sanctions over its weapons programmes from the United Nations Security Council, US, South Korea and others.
Heightened tensions in 2017 were followed by two years of nuclear diplomacy between Pyongyang and Washington, including three meetings between Kim and US President Donald Trump, but little tangible progress was made.
