South Korea's Joint Chiefs of Staff says Pyongyang appears to have fired a submarine-launched ballistic missile, or SLBM.
They say it was launched from the east coast of the country. It reportedly flew about 450 kilometers, and reached an altitude of about 910 kilometers.
The South Korean and US militaries are gathering more information. If confirmed, it would be the first launch of an SLBM by North Korea in three years.
Japan's Defense Minister Taro Kono says Tokyo is also analyzing whether it was an SLBM. He says it fell into the Sea of Japan, within Japan's exclusive economic zone. The Defense Ministry says it's the first time that's happened in nearly two years.
He said, "The missile launch was a great threat to the country's security. It is also really problematic and dangerous for aircraft and ships because missiles could hit planes or fall into the sea without notice."
Japan's Chief Cabinet Secretary Yoshihide Suga says no ships or aircraft in the area were damaged. Earlier, he said there may have been two projectiles, but that was later revised.
Prime Minister Shinzo Abe spoke to reporters about the incident on Wednesday morning.
He said, "Ballistic missile launches are a breach of UN resolutions. Japan severely condemns and strongly criticizes the act."
North Korea has conducted missile launches many times in recent months. Some are thought to involve short-range ballistic missiles.
The latest launch comes a day after North Korean state media said Pyongyang and Washington had agreed to resume working-level talks for the first time in months. The launch may be an attempt to see how the US reacts.
On Monday, North Korea's ambassador to the UN said it's up to Washington whether any progress will be made in denuclearization talks.
Pyongyang has said it wants relief from US sanctions as part of any negotiations. Washington wants the North to take strong steps toward denuclearization first.
US President Donald Trump and North Korean leader Kim Jong Un agreed to revive talks when they met at the demilitarized zone in June. That meeting came after the two leaders failed to reach an agreement on denuclearization in Vietnam in February.