Tokyo: Japanese Prime Minister Shinzo Abe said on Wednesday North Korea's latest missile launch was "unforgivable" and posed a grave threat to Japan's security, adding his government had lodged a stern protest against its isolated neighbour.

"This poses a grave threat to Japan's security, and is an unforgivable act that damages regional peace and stability markedly," Abe told reporters at the prime minister's residence.

North Korea fired a submarine-launched ballistic missile off its east coast early on Wednesday, South Korea's military said, the latest in a string of missile launches in defiance of UN Security Council resolutions.

The missile was fired at around 5.30am local time from near the coastal city of Sinpo, where satellite imagery shows a submarine base to be located, and travelled about 500 kilometres, officials at South Korea's Joint Chiefs of Staff and the Defence Ministry told Reuters.