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While North Korea is still struggling to develop nuclear-headed airborne missiles, it is fast increasing its underwater military power. The country possess numerous submarines, which it has used with devastating effect in the past, with fears they are now being adapted to fire nuclear weapons. Even more worrying is the submarines’ ability to 'disappear' and avoid detection. In 2015 around 50 went missing, setting alarm bells ringing in Seoul and Tokyo.

REUTERS Kim Jong-un may be preparing a nuclear strike using a secret submarine

Security expert Bruce Klingner told CNBC: “We didn't know where they were at the time. One would hope that we would keep very close tabs on those that could launch the submarine-launched ballistic missiles [SLBMs]. “All of that is very worrisome because they may very well have a nuclear weapon someday.” READ MORE: What will Donald Trump do about North Korea's nuclear weapons?

Inside North Korea: The pictures Kim Jong-un doesn't want you to see Thu, March 8, 2018 Photographer Eric Lafforgue ventured to North Korea six times. Thanks to digital memory cards, he was able to save photos that was forbidden to take inside the segregated state Play slideshow Eric Lafforgue/Exclusivepix Medi 1 of 69 Taking pictures in the DMZ is easy, but if you come too close to the soldiers, they stop you

Being able to launch an attack from a submarine, rather than a static site somewhere in North Korea, opens up Kim Jong-un’s enemies to a new sort of risk. Mr Klinger explained an attack from the ocean would take South Korea by surprise, due to the country’s defences geared towards an attack over the border. He said: “The problem with the SLBM is that it exposes South Korea's flanks to attack.”

REUTERS North Korea have already used submarines to kill dozens of enemy sailors

Submarines have already been used by the North Korean military to deadly effect, most recently in 2009 when a North Korea submarine sunk a South Korean navy vessel, killing 46 people. Adding nuclear weaponry to the mix would prove disastrous to country’s across east Asia.

REUTERS North Korea has around 50 submarines and is feared to be developing them for nuclear war

Other military experts warned of the devastating North Korea could cause using nuclear submarines. Nicholas Eberstadt of the American Enterprise Institute think tank said: “Their conventional forces may be not very capable at the moment, but they have a lot of weapons of mass destruction.

Forbidden photographs depict the true North Korean army Wed, April 12, 2017 While tourists are able to visit North Korea as part of a controlled tour group, they are asked by their handlers to never take photographs of soldiers. These pictures taken by photographer Eric Lafforgue show the reality of military service in the secretive state Play slideshow ERIC LAFFORGUE/EXCLUSIVEPIX MEDI 1 of 26 Soldiers enjoy a ride at the funfair in Pyongyang

Even if North Korea would end up losing a war, they could kill an awful lot of people on our side Nicholas Eberstadt, American Enterprise Institute

“Even if they would end up losing a war, they could kill an awful lot of people on our side.” It comes as tensions on the Korean peninsula continues to rise following the US military strike in Syria last week.

REUTERS Kim Jong-un looking out from one of North Korea's submarines