Pyongyang has vowed a "physical response" to plans to deploy an advanced US missile defence system in the South
North Korea has test-fired a ballistic missile which travelled 1,000km (620 miles) before landing in Japanese waters, the South and Japan say.
The missile was launched off the North's east coast early on Wednesday.
The North is barred from developing nuclear and ballistic missile technology by UN resolution.
But it had vowed a "physical response" to the US and Seoul's plan to deploy an advanced missile defence system in South Korea.
It has carried out repeated launches in recent months.
The US Strategic Command said two missiles had been fired simultaneously on Wednesday and that one exploded immediately after launch.
'Ambition to attack'
Wednesday's missile flight is believed to be the longest yet for a North Korean test.
The launch showed the North's "ambition to attack neighbouring countries", an official at South Korea's Joint Chiefs of Staff said. He added that the missile appeared to have been a medium-range Rodong.
The Japanese defence ministry said the missile landed inside its exclusive economic zone (EEZ), the 200-nautical mile of ocean around a country over which it has jurisdiction.
Prime Minister Shinzo Abe said it posed a grave threat to Japan's security, calling it an "unforgiveable act of violence". He said Tokyo had protested strongly against it.
The US similarly condemned the launch.
"We remain prepared to work to respond to further DPRK [Democratic People's Republic of Korea] provocations, as well as to defend ourselves and our allies from any attack or provocation," said State Department spokeswoman Anna Richey-Allen.
In June, after what appeared to be several failed launches, North Korea sent a mid-range missile more than 1,400km into the atmosphere, indicating it had made progress in its abilities to strike US targets in the region, according to analysts.
In July, North Korea fired three ballistic missiles into the sea, saying it was a simulated strike on the South.
Local media reports say South Korean officials believe the North is preparing to conduct a fifth test of a nuclear weapon.
But North Korea is not yet believed to have the ability to mount a nuclear warhead on a missile to make a deliverable weapon.
