Pyongyang has fired a missile over Japan's northern island of Hokkaido. Japan's government says it might have been an intermediate-range ballistic missile.
It split into three pieces before splashing into the Pacific Ocean Tuesday morning. There are no reports of damage.
South Korea's military says the missile reached an altitude of 550 kilometers and flew about 2,700 kilometers.
It suggests it was fired from close to the Pyongyang International Airport.
Tensions between North Korea and the US have been high, after the American president ramped up his rhetoric against the reclusive country.
On top of that, US and South Korean forces are conducting joint military exercises. They began last week and are scheduled to continue until Thursday.
Both countries have been stepping up readiness ahead of the North's National Foundation Day in early September. Experts say that the anniversary could mean more tests or launches.
This is the 5th time North Korea has flown a missile over Japan.
The Japanese government issued an emergency warning, calling on residents in Hokkaido to take shelter.
Japanese Prime Minister Shinzo Abe says North Korea's latest missile launch is an unprecedented, serious and grave threat for Japan.
He was speaking to reporters after a ministerial meeting of the National Security Council.
Abe said the Japanese government had closely tracked the missile's entire flight after it was launched early on Tuesday. He said the government had taken every imaginable measure to ensure the safety of Japanese citizens.
The prime minister said that the government lodged a strong protest to Pyongyang. He also said Japan will ask the UN Security Council to convene an emergency meeting, and call on the international community to put more pressure on the North.
Abe added that the government will do its utmost to guarantee the safety of Japanese citizens under a strong Japan-US alliance.