North Korea has fired a submarine-launched ballistic missile off its east coast, according to South Korea's military, the latest in a string of missile launches by Pyongyang in defiance of UN Security Council resolutions.

The missile was test-fired at around 5:30am (08:30 GMT) on Wednesday near the coastal city of Sinpo, where satellite imagery shows a submarine base is located, an official at South Korea's Joint Chiefs of Staff said.

Yoshihide Suga, Japan's Chief Cabinet Secretary, said the projectile reached Japan's air defence identification zone - an area of control designated by countries to help maintain air security.

The launch comes two days after South Korea and the US began annual military exercises that North Korea condemned as a preparation for invasion. It has threatened retaliation.

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China's Xinhua news agency said the launch could be seen as a response to the drills, calling the exercises "a dangerous game".

North Korea has become further isolated after a January nuclear test, its fourth, and the launch of a long-range rocket in February brought tightened UN sanctions.

It has launched numerous missiles of various types this year, including one this month that landed in or near Japanese-controlled waters.

Tensions on the Korean peninsula were exacerbated by the recent defection of North Korea's deputy ambassador in London to South Korea, an embarrassing setback to the regime of North Korean leader Kim Jong-un.