North Korea fired four ballistic missiles early on Monday, three of which landed in Japan's exclusive economic zone, Japanese prime minister Shinzo Abe said, the latest in a series of provocative tests by the reclusive state.
"Multiple ballistic missiles" were launched from the Tongchang-ri region near the North's border with China and flew about 1,000 km (620 miles), South Korean military officials said, without providing the number of missiles.
South Korea's military confirmed Japan's report.
"South Korea and the United States are conducting a close-up analysis, regarding further information," South Korea's office of joint chiefs of staff said in a statement. Acting president Hwang Kyo-ahn convened a national security meeting, South Korea's presidential office said in a text message.
Japanese officials described the launches as a grave threat and said they lodged "strong protests" with nuclear-armed North Korea.
"The launches are clearly in violation of Security Council resolutions. It is an extremely dangerous action," Abe said during lawmaker questions in parliament.
No reports of damage to shipping or aircraft had been received since the launches, Japanese officials said.
The US military did not immediately comment. A US official, speaking on condition of anonymity, said the United States also detected apparent launch activity in North Korea but declined to offer details.
North Korea had threatened to take "strong retaliatory measures" after South Korea and the United States began annual joint military drills on Wednesday that test their defensive readiness against possible aggression from the North.
North Korea criticises the annual drills calling them preparation for war against it.
Last year, North Korea fired a long-range rocket from Tongchang-ri that put an object into orbit. The launch was condemned by the United Nations for violating resolutions that ban the use of missile technology.
"Not only Pukguksong-2 but newer independent strategic weapons will fly high vigorously in the sky off the ground as long as the United States and the puppet regime are going ahead with their nuclear threat to us and an exercise for invasion war against the North," North Korea's Rodong Sinmun newspaper, the mouthpiece of the ruling Workers' Party said in a commentary last week.
The United States has 28,500 troops stationed in South Korea as a deterrent against a potential aggression from the North