North Korea fires Submarine-Launched Ballistic Missile off coast of South Korea

North Korea has fired a submarine-based ballistic missile off the coast of South Korea leaving the two countries on a war alert.

The incident comes as Pyongyang ramps up military tensions between North and South with a series of armed provocations over the past 12 months.

The latest missile launch, in the Sea of Japan, was announced by South Korea's Yonhap news agency.

North Korea appeared to resume activities this year aimed at producing plutonium which can be used in the core of an atomic bomb the U.N. nuclear watchdog has confirmed, although it added that signs of those activities stopped last month.

The North has even threatened to turn the US and South Korean armies into a "heap of ashes" after they were spotted performing joint military exercises.