The missile tests prompted S Korea's leader to call for the quick deployment of a controversial US anti-missile system known as THAAD. The US condemned the DPRK tests.
Nuclear-armed North Korea fired off four ballistic missiles during the early hours of Monday morning.
Japan claims that three of them landed within its waters.
According to South Korea, the missiles were launched from a DPRK facility near the Chinese border, and travelled around 1,000 kilometres.
Reacting to North Korea's action, Japanese Prime Minister Shinzo Abe said, "We will not tolerate the launches, which are clearly in violation of United Nations Security Council resolutions. We have strongly rebuked North Korea."
The launch prompted South Korea's acting leader to call for quick deployment of the American Terminal High Altitude Area Defense (THAAD) anti-missile system, a move that has become not only a tension point with North Korea, but also China.
China is worried the system's radar may be used by the US to spy on its military.
Washington DC and Seoul say that THAAD is strictly aimed at protecting against threats from North Korea.
The US also condemned Monday's missile tests. The State Department 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."
North Korea threatened to take "strong retaliatory measures" after joint US-South Korean defence drills that kicked off last week, an annual event that Pyongyang calls preparation for war.
The DPRK has staged regular test launches in the past year sparking concern in Washington DC that it plans to develop an intercontinental ballistic missile that could reach the United States.