North Korea has test-fired another ballistic missile into the sea, South Korea's defence military said.

 The missile was launched off the North's east coast, according to the Joint Chiefs of Staff.

 Pyongyang has vowed to take a "physical response" to the US and Seoul's plan to deploy an advanced missile defence system in South Korea.

North Korea is prohibited from developing nuclear and ballistic missile technology by UN resolutions.

 But it has carried out repeated launches in recent months.

The launch showed the North's "ambition to attack neighbouring countries", an official at the Joint Chiefs of Staff said. He added that the missile, which appeared to be a medium-range Rodong, flew for about 1,000km (620 miles).

Japan's PM Shinzo Abe said the test launch posed a grave threat to the country's security and that Tokyo had protested strongly against it.

The US similarly condemned the launch.

"We remain prepared to work to respond to further DPRK provocations, as well as to defend ourselves and our allies from any attack or provocation," said State Department spokeswoman Anna Richey-Allen.

 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.