North Korea has fired four ballistic missiles into the sea off Japan's northwest coast, South Korean and Japanese officials say.
It comes just days after the reclusive state promised retaliation over US-South Korea military drills it sees as a preparation for war.
South Korea's military said the missiles were unlikely to have been intercontinental ballistic missiles (ICBM) which could reach the United States, but flew on average 1,000 km and reached a height of 260 km.
Some of the missiles landed in waters as close as 300 km to Japan's northwest coast, Japan's Defence Minister Tomomi Inada said in Tokyo.
"The launches are clearly in violation of (UN) Security Council resolutions. It is an extremely dangerous action," Japanese Prime Minister Shinzo Abe said in parliament, adding "strong protests" had been lodged with nuclear-armed North Korea.
South Korea's acting President Hwang Kyo-ahn condemned the launches as a direct challenge to the international community and said the country would swiftly deploy a US anti-missile defence in the face of angry objections from China.
The missiles were launched from the Tongchang-ri region near the North's border with China, South Korean military spokesman Roh Jae-cheon told a briefing. It was too early to say what the relatively low altitude indicated about the types of missiles, he added.
"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.
The US military said it detected and tracked what it assessed was a North Korean missile launch, but it did not pose a threat to North America.
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.
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 ballistic missile technology.
North Korea test fired a new type of missile into the sea early last month, and has said it will continue to launch new strategic weapons.
The United States has about 28,500 troops and equipment stationed in the South, and plans to roll out the THAAD (Terminal High Altitude Area Defense) anti-missile defence system in the country by the end of the year.
China later criticised the missile test and urged North Korea, South Korea and the United States to remain calm.
"China opposes North Korea's violation of the UN Security Council's stipulation," Geng Shuang, a Chinese Foreign Ministry spokesman, said.
