The latest missile test has alarmed Tokyo, which has expressed its outrage, while also ramping up tensions with South Korea and the United States.
North Korea launched a ballistic missile on Wednesday that landed in or near Japanese-controlled waters for the first time, the latest in a series of launches by the isolated country in defiance of United Nations Security Council resolutions.
The latest launch was an apparent show of force against the planned deployment of a US missile defence system in the South and comes ahead of a US-South Korea joint military exercise scheduled for later this month.
UN resolutions prohibit North Korea from developing ballistic missile technology. Wednesday's test drew an outraged response from alarmed Tokyo and ramped up tensions with the United States and South Korea.
Japan's Prime Minister Shinzo Abe on Wednesday condemned the North Korean test missile that landed 250 kilometres off Japan's coast as an "outrageous act" which threatened his country.
"It's a serious threat against our country's security," Abe told reporters. "This is an outrageous act that cannot be tolerated."
Japanese Defence Minister Gen Nakatani said the missile landed in the Sea of Japan off the north coast in the country's Exclusive Economic Zone (EEZ), the first time a North Korean missile has landed in Japan's EEZ since 1998.
It was the first ever North Korean missile to land in Japan's EEZ in the Sea of Japan (East Sea) facing the Korean peninsula. A North Korean missile in 1998 landed in Japan's EEZ in the Pacific Ocean after having flown over the country's territory.
Chief cabinet secretary Yoshihide Suga, the government's top spokesman, also harshly criticised the launch.
"There was no early warning," he told reporters. "From the perspective of the safety of aircraft and ships, it is an extremely problematic, dangerous act," he added.
Japan also said its self-defense force would remain on alert in case of further launches.
The missile that landed in the Sea of Japan was launched at about 7:50 am Seoul time from a region in South Hwanghae province to the southwest of the capital, Pyongyang, South Korea's Office of the Joint Chiefs of Staff said in a statement.
The launch showed North Korean ambition to "directly and broadly attack neighboring countries and target several places in the Republic of Korea such as ports and airfields," the statement said, referring to South Korea by its official title.
The missile appeared to be a Rodong-type medium-range missile that flew about 1,000 km, the statement added.
South Korean Foreign Minister Yun Byung-Se said Wednesday's missile test only served to "underline the need to deploy the Terminal High Altitude Area Defence (THAAD) anti-missile system."
North Korea had threatened a "physical response" against the deployment decision announced last month.
Tensions on the divided Korean peninsula are also building up ahead of an annual South Korea-US military exercise later this month that involves tens of thousands of troops.
North Korea says such drills are a provocative rehearsal for invasion, while Washington and Seoul insist they are purely defensive in nature.
The United States issued a stern warning on Tuesday saying that it was prepared to "defend ourselves and our allies".
"We are aware of reports that the DPRK (North Korea) fired ballistic missiles," said State Department spokeswoman Anna Richey-Allen, noting that Washington strongly condemned this and other recent tests.
"We remain prepared to work with our allies and partners around the world to respond to further DPRK provocations, as well as to defend ourselves and our allies from any attack or provocation."
Richey-Allen said the US commitment to defending allies like South Korea and Japan remains "ironclad."
"We intend to raise our concerns at the UN to bolster international resolve in holding the DPRK accountable for these provocative actions," the spokeswoman added.
Meanwhile, US Strategic Command said that two Rodong intermediate-range missiles were simultaneously launched from a site in western North Korea at around 7:50am Seoul time.
The Rodong is a scaled-up Scud variant with a maximum range of around 1,300 kilometres.
"Initial indications reveal one of the missiles exploded immediately after launch, while the second was tracked over North Korea and into the Sea of Japan," it said in a statement.
The United States condemned what it called a clear violation of UN Security Council resolutions explicitly prohibiting North Korea's use of ballistic missile technology.
"This provocation only serves to increase the international community's resolve to counter (North Korea's) prohibited activities," said Pentagon spokesman Gary Ross.
"Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, is ironclad," Ross said on Wednesday.
"We remain prepared to defend ourselves and our allies from any attack or provocation," he added.
There are nearly 30,000 US troops permanently stationed in South Korea.
Pyongyang has repeatedly warned of pre-emptive nuclear strikes against the South and US targets there and elsewhere, although the main focus of its nuclear weapons programme is to develop a credible strike threat against the US mainland.
Since its fourth nuclear test, North Korea has claimed a series of technical breakthroughs for its weapons programme.
It said it had miniaturised a nuclear warhead and successfully tested an engine designed for an inter-continental ballistic missile that could reach the US mainland.
While some experts say the claims are exaggerated, most acknowledge that the North's nuclear and ballistic missile programmes have made significant strides.
Wednesday's test follows the launch of three ballistic missiles on July 19, including one Rodong, in what the North said were simulated nuclear strikes on the South.
North Korea came under the latest round of UN Security Council sanctions in March after its fourth nuclear test in January and the launch of a long-range rocket the following month.
Tensions have been high on the Korean peninsula since the January nuclear test. The two Koreas remain technically at war under a truce that ended fighting in the 1950-53 Korean War.