North Korea fired a ballistic missile over Japan's northern Hokkaido island into the sea early on Tuesday, prompting warnings to residents to take cover and drawing a sharp reaction from Japanese Prime Minister Shinzo Abe.
The test, one of the most provocative ever from the reclusive state, came as U.S. and South Korean forces conduct annual military exercises on the peninsula, angering North Korea which sees them as a preparation for invasion.
North Korea has conducted dozens of ballistic missile tests under young leader Kim Jong Un, the most recent on Saturday, in defiance of U.N. sanctions, but firing projectiles over mainland Japan is rare.
"North Korea's reckless action is an unprecedented, serious and a grave threat to our nation," Japanese Prime Minister Shinzo Abe told reporters.
Abe said he spoke to U.S. President Donald Trump on Tuesday and they agreed to increase pressure on North Korea. Trump also said the United States was "100 percent with Japan", Abe told reporters.
The U.S. disarmament ambassador said Washington still needed to do "further analysis" of the launch, but it will be the subject of a U.N. Security Council meeting later in the day.
"It's another provocation by North Korea, they just seem to continue to happen," U.S. envoy Robert Wood told reporters in Geneva.
"This is a big concern of course to my government and to a number of other governments," Wood said before a session of the U.N.-sponsored Conference on Disarmament.