It is the third round of missile launches by North Korea this week, extending a record pace in weapons testing as it accelerates a push to expand its arsenal and pressure Washington to accept it as a nuclear power.
North Korea has fired two more ballistic missiles, Seoul's military said, Pyongyang's third such launch in less than a week and just hours after US Vice President Kamala Harris left South Korea.
The South's military said on Thursday it had detected the launch of "two short-range ballistic missiles from the Sunchon area in South Pyongan province".
"Our military has reinforced monitoring and surveillance and is maintaining utmost readiness in coordination with the US," Seoul's Joint Chiefs of Staff said in a statement.
Japan's coast guard also confirmed a likely ballistic missile launch from North Korea, citing information from the defence ministry.
Public broadcaster NHK said the projectile "appears to have fallen outside Japan's Exclusive Economic Zone", citing unnamed sources from the defence ministry.
While in South Korea, Harris toured the country's heavily fortified border with the nuclear-armed North, part of a trip aimed at strengthening the security alliance with Seoul.
Speaking at the Demilitarized Zone (DMZ), Harris said the US commitment to South Korea's defence was "ironclad", adding that the allies were "aligned" in their response to the growing threat posed by the North's weapons programmes.
Record-breaking streak of tests
Washington has about 28,500 troops stationed in South Korea to help protect it from the North, and the allies are conducting a large-scale joint naval exercise this week in a show of force.
Pyongyang conducted two banned ballistic missile launches in the days before Harris's arrival, continuing a record-breaking streak of weapons tests this year.
The North fired a short-range ballistic missile (SRBM) on Sunday and two SRBMs on Wednesday, Seoul and Tokyo said.
Under the South's hawkish new President Yoon Suk-yeol, Seoul and Washington have boosted joint military exercises, which they insist are purely defensive.
North Korea sees them as rehearsals for an invasion.
Seoul announced on Thursday that it would hold trilateral anti-submarine drills with Japan and the US, the first such exercises since 2017.
South Korean officials said this weekend they had detected signs Pyongyang could be preparing to fire a submarine-launched ballistic missile.