North Korea has fired a submarine-based ballistic missile, the Pentagon has told the BBC.
The KN-11 missile was launched off the coast of Sinpo and flew about 500km (300 miles) before falling into the Sea of Japan, a US official said.
South Korea's Yonhap news agency said the missile had fallen inside Japan's Air Defence Identification Zone.
North Korea, believed to be developing nuclear weapons, is banned by the UN from any use of ballistic missiles.
The reported test follows an attempted launch last month that South Korea said failed in its initial flight stage.
It took place as foreign ministers from Japan, China and South Korea were meeting in Tokyo.
Earlier, North Korea warned that current joint military exercises by South Korea and the US - which it sees as a rehearsal for invasion - were pushing the Korean peninsula towards the brink of a war.
Last month, the US and South Korea also agreed to deploy a missile defence system to counter threats from the North.
In January, North Korea conducted its fourth nuclear test, claiming it to be its first using a hydrogen bomb.
Shortly after that Pyongyang launched a satellite, widely seen as a test of long-range missile technology.