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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.

(Image: Reuters)

Pyongyang vowed in 2013 to restart all nuclear facilities, including the main reactor at its Yongbyon site that had been shut down and has been at the heart of its weapons programme.

It said last year that Yongbyon was operating and that it was working to improve the “quality and quantity” of its nuclear weapons. It has since carried out what is widely believed to have been its fourth nuclear test.

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