North Korea is reported to have fired a ballistic missile from its east coast into the Sea of Japan.
South Korea's defence ministry said the missile had flown around 60km.
The US military confirmed the missile test, adding that "it did not pose a threat to North America".
US Pacific Command said the launch involved a KN-15 medium-range ballistic missile.
It comes a day before a summit between Chinese President Xi Jinping and US President Donald Trump, who are set to discuss Pyongyang's arms programme.
Any use of ballistic missile technology by North Korea is a violation of UN Security Council resolutions.
"The launch took place possibly in consideration of the US-China summit, while at the same time it was to check its missile capability," a South Korean official said.
The missile was fired at a high angle and reached an altitude of 189km, the official added.
The launch drew swift condemnation from Japan, with Prime Minister Shinzo Abe saying further provocative action was possible.
Japan's Chief Cabinet Secretary Yoshihide Suga described the launch as "extremely problematic" and said Tokyo had lodged a strong protest.
South Korea's foreign ministry also condemned the launch as a blunt challenge to a series of UN Security Council resolutions targeting North Korea's nuclear and missile programme. Seoul called a National Security Council meeting and vowed to respond strongly in case of further provocations.
"The United States has spoken enough about North Korea. We have no further comment," US Secretary of State Rex Tillerson said in a terse statement.
