North Korea has fired four banned ballistic missiles over 620 miles, with three of them landing in waters which Japan claims as its exclusive economic zone, officials said.
The test launches appear to be a reaction to huge US-South Korean military drills which those countries consider routine, but are viewed as an invasion rehearsal by Pyongyang.
It is not clear what exact type of missile was fired, but the tests will be viewed as a provocation by the Trump administration, which is working on its policy for North Korea.
The New York Times reported over the weekend that the US still cannot effectively counter Pyongyang's actions despite efforts to perfect cyber and electronic strikes against North Korea's missile programme.
Pyongyang has test-launched a series of missiles of various ranges in recent months, including a new intermediate-range missile in February.
It also conducted two nuclear tests last year. The ramped-up tests come as leader Kim Jong Un pushes for a nuclear and missile programme which can deter what he calls US and South Korean hostility toward the North.
There have been widespread worries that the North will conduct an ICBM test that, when perfected, could in theory reach the US mainland. Washington would consider such a capability a major threat.
US national security adviser HR McMaster and his South Korean counterpart Kim Kwan-jin have condemned the launches and agreed to boost co-operation to bring more effective sanctions and pressure to bear on the North, according to South Korea's presidential office.
Japanese prime minister Shinzo Abe said three missiles landed in the 200-nautical-mile offshore area where Tokyo has sovereign rights for exploring and exploiting resources.
He said a fourth missile fell "near" Japan's exclusive economic zone.
This is the third time that North Korean missiles have fallen in the Japanese zone, beginning last August. Japanese leaders see the launches into nearby waters as a growing threat.
European Union foreign affairs chief Federica Mogherini said the launches were "in utter disregard" of several UN resolutions and that the EU would consult with Japan and international partners on how to react.
She also said North Korea needed to immediately halt plans for more such missile launches.
South Korea's joint chiefs of staff said in a statement that the launches were made from the Tongchang-ri area in North Pyongan province.
The area is the home of the North's Sohae rocket launch site where it has conducted prohibited long-range rocket launches in recent years.
US state department spokesman Mark Toner said: "We remain prepared - and will continue to take steps to increase our readiness - to defend ourselves and our allies from attack, and are prepared to use the full range of capabilities at our disposal against this growing threat."
An unidentified spokesman for the North's general staff of the Korean People's Army said last week that Pyongyang's reaction to the southern drills would be the "toughest ever".
The US has 28,500 troops stationed in South Korea and 50,000 in Japan, as a deterrent against potential aggression from the North.