North Korea has test-fired a ballistic missile which landed in Japanese waters, South Korea and Japan say.
The missile was launched off the North's east coast early on Wednesday and travelled about 1,000km (620 miles), one of the North's longest launches to date.
Japanese PM Shinzo Abe said it was a "grave threat" to his country.
The North is barred from developing nuclear and ballistic missile technology by UN resolution.
But it had vowed a "physical response" after the US and South Korea agreed plans to deploy an advanced US missile defence system in South Korea.
The North has carried out repeated missile tests in recent months.
'Ambition to attack'
The US Strategic Command said two missiles had been fired simultaneously on Wednesday from Hwanghae province at about 07:50 Seoul time (22:50 GMT Tuesday), but one exploded immediately after launch.
The Japanese defence ministry said the other missile landed inside its exclusive economic zone (EEZ) - the 200 nautical miles of ocean around a country over which it has jurisdiction.
An official at South Korea's Joint Chiefs of Staff said it appeared to have been a medium-range Rodong.
The launch showed the North's "ambition to attack neighbouring countries", he said. There were no reports of any damage.
Mr 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.