Nuclear-armed North Korea launched four ballistic missiles on Monday in another challenge to President Donald Trump, with three landing provocatively close to America's ally Japan.
Seoul and Washington began annual joint military exercises last week that always infuriate Pyongyang, with the North's military warning of "merciless nuclear counter-action".
Under leader Kim Jong-Un, Pyongyang has ambitions to develop an intercontinental ballistic missile (ICBM) capable of reaching the US mainland -- which Trump has vowed will not happen.
Seoul said four missiles were fired from North Pyongan province into the East Sea -- its name for the Sea of Japan -- and that South Korea and the US were "closely analysing" tracking data for further details.
The missiles travelled around 1,000 kilometres (620 miles) and reached an altitude of 260 kilometres, said a spokesman for South Korea's Joint Chiefs of Staff, adding they were unlikely to be ICBMs.
Japanese Prime Minister Shinzo Abe said three of the North Korean missiles came down in Tokyo's Exclusive Economic Zone (EEZ) -- waters extending 200 nautical miles (370 kilometres) from its coast.
"This clearly shows North Korea has entered a new stage of threat," Abe said in parliament.
"Repeated launches by North Korea are a serious, provocative action in terms of security and clearly violate UN security council resolutions. We can never tolerate this," he said.
Pyongyang carried out two atomic tests last year and a series of missile launches, but Monday was only the second time its devices entered Japan's EEZ.
After an emergency meeting of South Korea's National Security Council, acting president Hwang Kyo-Ahn called the North's nuclear and missile provocations "immediate and real threats" to his country.
"Considering the North Korean leadership's brutality and recklessness shown through the murder of Kim Jong-Nam, the results of the North having a nuclear weapon in its hands will be gruesome beyond imagination," he said.
Seoul has blamed Pyongyang for the killing of the half-brother of the North's leader by two women using VX nerve agent at Kuala Lumpur's international airport last month.
Hwang called for "swift deployment" of a US missile defence system, THAAD, a proposal which has infuriated neighbouring China, the North's key diplomatic protector and main provider of trade and aid.
Seoul said its armed forces were "closely monitoring the North's military for further provocations and maintaining military readiness".