Japan's Coast Guard also reported a suspected ballistic missile launch by North Korea.
North Korea fired three ballistic missiles towards the Sea of Japan early Wednesday, Seoul's military said, just one day after President Joe Biden wrapped up his first Asia visit as US leader.
South Korea's Joint Chiefs of Staff said in a statement that it had "detected at around 0600 (2100 GMT), 0637 and 0642 the firings of ballistic missiles launched from Sunan area."
Japan's coastguard warned of a "possible ballistic missile" launch from North Korea, telling vessels to stay away from fallen objects in the waters.
Japanese Prime Minister Fumio Kishida told reporters that Tokyo was trying to confirm information about the launch.
South Korea's new President Yoon Suk-yeol will oversee a meeting of the National Security Council at 0730 to discuss the launches, his office said.
Yoon, who was sworn in earlier this month, has vowed to get tough with Pyongyang after five years of failed diplomacy.
The Wednesday launches are the latest in a blitz of sanctions-busting weapons tests by Pyongyang this year, including test-firing intercontinental ballistic missiles at full range for the first time since 2017.
The latest apparent test come just days after Biden left South Korea Sunday after a trip overshadowed by US officials warning that Pyongyang's leader Kim Jong Un could carry out a nuclear test while Biden was in the region.
While in South Korea, Biden joined Yoon for talks, including discussing expanded military exercises to counter Kim's sabre rattling.
Joint exercises had been scaled back due to Covid and in order for Biden and Yoon's predecessors, Donald Trump and Moon Jae-in, to embark on a round of high-profile but ultimately unsuccessful diplomacy with North Korea.
Any build-up of forces or expansion of joint military exercises would likely enrage Pyongyang, which views the drills as rehearsals for invasion.
On his last day in Seoul, Biden told reporters he had a only a short message for Kim: "Hello. Period."
And he added that the United States was "prepared for anything North Korea does."
- Covid and missiles -
Kim has recently doubled down on his programme of military modernisation.
Despite struggling with a recent Covid-19 outbreak, new satellite imagery has indicated the North has resumed construction at a long-dormant nuclear reactor.
Earlier this month, North Korea confirmed its first ever Omicron cases in Pyongyang, and the virus has since torn through its unvaccinated population of 25 million.
More than three million people have been sick with "fever" North Korean state media said Wednesday, with 68 deaths since the outbreak began in late April.
How that crisis might impact Kim's decision on nuclear tests is one of the many unknowns that US and South Korean officials have been weighing.
On May 12, North Korea test-fired ballistic missiles the same day that leader Kim declared an "emergency" over the Covid outbreak.
A few days earlier, North Korea test-fired a submarine-launched ballistic missile, which itself came just three days after a separate ballistic missile launch.
North Korea's state media, which typically reports on weapons tests within 24 hours of a successful launch, has not commented on any of those tests.
South Korea said last week that North Korea's preparations for a nuclear test had been completed, and they were waiting for the right time.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)