North Korea has launched a missile, with reports suggesting it has landed in the ocean.
Japanese broadcaster NHK said it appears the missile flew northern Japan. The government warned people to "stay away from anything that could be missile debris".
The launch was quickly confirmed by the South Korean military, which later said it fell into the ocean after travelling about 3700km from its Pyongyang launch site.
"North Korea fired an unidentified missile eastward from the vicinity of Pyongyang this morning," the Joint Chiefs of Staff said in a statement.
It reached an estimated altitude of 770km, the Japanese government saying it hit the water about 2000km of the east cape of Hokkaido.
"These repeated provocations on the part of North Korea are unpermissible and we protest in the strongest words," said Japan Chief Cabinet Secretary Yoshihide Suga.
The launch comes after North Korea threatened to "sink" Japan and reduce the US to "ashes and darkness".
"The four islands of the archipelago should be sunken into the sea by the nuclear bomb of Juche. Japan is no longer needed to exist near us," Pyongyang's Korea Asia-Pacific Peace Committee said in a statement broadcast on state TV.
"Let's reduce the US mainland into ashes and darkness. Let's vent our spite with mobilisation of all retaliation means which have been prepared till now."
South Korean President Moon Jae-in is to hold an emergency meeting with his officials, as is the Japanese government.
"This is another dangerous, reckless, criminal act by the North Korean regime, threatening the stability of the region and the world and we condemn it, utterly," said Australian Prime Minister Malcolm Turnbull.
"This is a sign, I believe, of their frustration at the increased sanctions on North Korea, recently imposed by the Security Council. It's a sign that the sanctions are working."