Kim Jong-Un 'Miniaturises Nuclear Weapons'

North Korea claims it has miniaturised nuclear weapons, making the announcement as it axed UN Secretary General Ban Ki-moon's visit one day before he was due to arrive.

If the arms advance is true, it means the country can mount nuclear warheads into the tips of its ballistic weapons.

"It has been a long time since we began miniaturising and diversifying our means of nuclear strike," Pyongyang's National Defence Commission (NDC) announced.

"We have also reached the stage where the highest accuracy rate is guaranteed not only for short and medium-range missiles but long-range missiles as well. We don't hide this fact."

It came two weeks after North Korea said it had launched a ballistic missile from a submarine, allowing it to fire nuclear weapons far beyond the Korean peninsula.

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The country's Supreme Leader Kim Jong-Un was pictured grinning and pointing at a missile emerging from the waves - however, some analysts believe the images had been manipulated.

Mr Kim cancelled the UN chief's visit after accusing him of fuelling regional tensions.

Mr Ban, who had urged the North to avoid actions that could escalate military tensions, was due to visit the Kaesong industrial zone on Thursday.

"No explanation was given for this last-minute change," the former South Korea foreign minister told a news conference in Seoul. "This decision by Pyongyang is deeply regrettable."

If the visit had gone ahead, he would have been the first UN secretary-general to set foot in the communist state for more than 20 years.

The NDC, chaired by Mr Kim, said the submarine-launched ballistic missile test "represents a higher stage in our efforts to develop strategic striking forces".

"The United States and its cronies must stop kicking up a ruckus over our measures to strengthen self-defence capabilities," it said.

The North has carried out three atomic tests and has long had nuclear weapons. But it is unclear whether it has the technology to deliver them by missile.

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Sky's Asia Correspondent Mark Stone said: "While any claim from North Korea should be read with caution given that they are prone to making exaggerated claims, the announcement will be of real concern not only to the Americans and South Koreans but also to the Chinese.

"For years, North Korea has been trying to miniaturise its existing nuclear weapon technology. If successful it can mount nuclear warheads into the tips of ballistic weapons which they already possess.

"Sources with knowledge of both South Korean and Chinese government thinking have told Sky News that both governments have become increasingly convinced that North Korea has been successful with its miniaturisation.

"In short, and if true, this development means that North Korea has the ability to deploy a nuclear-armed missile."

America's top homeland security commander, Admiral William Gortney, said last month the North is capable of mounting a miniaturised nuclear warhead on its new KN-08 intercontinental ballistic missile.

However, some experts believe the North is still "many years" from launching nuclear missiles from its submarine fleet.

German aerospace engineers Schmucker Technologie said photos of the 8 May launch were "strongly modified", pointing out that reflections of the exhaust flame in the sea do not line up with the missile.

North Korea, which regularly threatens to destroy the US and its allies, has a record of faking arms technology, including Photoshopped missiles at military parades in 2012 and 2013, the Munich-based company said.

It agreed with analysis by experts on websites 38north.org and armscontrolwonk.com that the missile was most likely launched from a submerged barge, and not a submarine.

However, South Korean military officials still believe the launch was authentic.

"We haven't changed our stance that the rocket was fired from a submarine and flew about 150 metres out of the water," a South Korean military official said.

"As we have previously said, the photos don't appear to have been manipulated."

Meanwhile, reports that Mr Kim executed his defence minister Hyon Yong Chol may not be true, according to South Korea's Yonhap news agency.

A documentary aired by the North's state broadcaster on Tuesday shows Mr Kim holding a baby with the 66-year-old defence chief applauding in the background.

It goes against the North's usual practice of deleting executed senior officials from footage, Yonhap said.

Mr Hyon was reportedly executed by anti-aircraft fire as punishment for falling asleep at a military event.