North Korea has test-fired fired two missiles, and the US and regional powers are trying to find out more details about the launch.
A senior US official first confirmed two ballistic missiles had been fired but gave no further details on their range or flight paths.
In Seoul, the joint chiefs of staff said in a statement that the "unidentified projectile" was launched into the Sea of Japan, known as the East Sea in Korea.
A short while later, the Japanese government said the missiles flew about 450km and landed outside the Japanese exclusive economic zone.
"The first launch in just less than a year represents a threat to peace and stability in Japan and the region and violates UN resolutions," Japanese Prime Minister Yoshihide Suga said in comments aired by public broadcaster NHK.
Japan's defence ministry also warned ships to watch for falling debris.
"North Korea this morning fired two unidentified projectiles into the East Sea from South Hamkyung Province," the South Korean government said in a statement.
"South Korea and US intelligence are analysing [the situation] for any additional information.
"While our military has strengthened awareness and monitoring … [officials] are closely working together and maintaining readiness.”
South Korea will convene an emergency meeting of its national security council to discuss the launches.
The launches came a day after US and South Korean officials confirmed that North Korea had fired short-range weapons presumed to be cruise missiles into its western sea last weekend.
The US tracks all North Korean weapons tests through radar and satellites and detects heat signatures of missiles almost as soon as they are launched.
The Japanese Coast Guard warned ships to be on the lookout for falling debris.
North Korea is banned from developing ballistic missiles under UN Security Council resolutions but it has made rapid progress in its capabilities under leader Kim Jong-un.
It has a long history of using weapons tests as a provocation in a carefully calibrated process to forward its objectives.
On Tuesday, the White House confirmed North Korea had test-fired missiles last weekend.
Administration officials, speaking anonymously, downplayed the missiles as "common" military testing and said they would not block Washington's efforts to engage with North Korea on denuclearisation.
Analysts took them as a modest challenge to the new administration as it tries to engage with Mr Kim in talks on ridding the peninsula of nuclear weapons.
The launches came days after US Secretary of State Antony Blinken and Defence Secretary Lloyd Austin visited Japan and South Korea to discuss their alliance and security issues in the region, with a nuclear North Korea regarded as a central threat.
They also followed joint exercises by US and South Korean defence forces between March 8 and 17.