N Korea blast ‘five times bigger than Nagasaki’

It added that the device was designed to be mounted on its newly developed intercontinental ballistic missile.

Earlier, Japan confirmed its near neighbour had conducted a sixth nuclear test.

Japan’s meteorological agency said the resulting tremors were at least 10 times as powerful as North Korea’s previous nuclear test, last September.

Experts estimated that blast to have been around 10 kilotons.

It means this latest device was about five times larger than the bomb dropped on Nagasaki during World War II in August 1945.

:: Key steps in North Korea’s nuclear and ballistic missile ambitions

Japan’s foreign minister, Taro Kono, described the new explosion as “extremely unforgivable”.

The Tokyo government has registered a protest with the North Korean embassy in Beijing, he said.

Hours earlier, Japanese Prime Minister Shinzo Abe spoke to President Trump on the phone about the “escalating” situation.

South Korea’s president, Moon Jae-in, has called for the “strongest” response.

The US Geological Survey said a magnitude 6.3 tremor struck near a weapons test site in the northeast of North Korea.

A second tremor was detected measuring 4.6, South Korea’s Yonhap news agency said.

Sky News’ Asia Correspondent, Katie Stallard, said: “The significant point to take out of all of this is in terms of what response we see from the United States.

“We’ve seen over the last weeks and months increasing rhetoric from both sides, culminating memorably in Donald Trump’s threat of ‘fire and fury’ against Kim Jong Un’s regime if they continued to make threats.

“So the question is, given his previous rhetoric, his previous threats, how Donald Trump plans to respond to this.”

It comes just hours after Pyongyang claimed to have developed a hydrogen bomb that could be loaded into a long-range missile.

The state-run Korean Central News Agency released pictures of Kim Jong Un visiting the country’s Nuclear Weapons Institute in Pyongyang.

Dressed in a black suit and flanked by his lieutenants, the Supreme Leader was seen examining a metal casing with two bulges that was apparently the purported weapon.

State media quoted Mr Kim as saying it was a “thermonuclear weapon with super explosive power” and “all components of the H-bomb were 100% domestically made”.

It claimed the hydrogen bomb’s power is adjustable to hundreds of kilotons and can be detonated at high altitudes.

Experts are sceptical about the claim that Pyongyang has mastered hydrogen technology, but it is almost impossible to independently confirm statements about its highly secret weapons programme.

Melissa Hanham, of the Middlebury Institute for International Studies in California, said the images released by the North could not be proved real.

“We don’t know if this thing is full of styrofoam, but yes, it is shaped like it has two devices,” she said on Twitter.

“It doesn’t need to be shaped like that on the outside, but they threw in a diagram, just so we would get the message.

“The bottom line is that they probably are going to do a thermonuclear test in the future. We won’t know if it’s this object though.”

:: North Korea threat: What does Kim Jong Un really want?

The developments are raising already high tensions on the Korean Peninsula and in Washington that the North is getting closer to its goal of an arsenal of intercontinental ballistic missiles (ICBMs) that can reach the US mainland.

Last month, US media reported that US intelligence officials had concluded Pyongyang had successfully miniaturised a nuclear weapon.

It prompted President Donald Trump to issue a warning of “fire and fury” on the North, while Pyongyang threatened to fire missiles towards the US territory of Guam.

On Tuesday, the North fired a mid-range ballistic missile that flew over Japan – a test considered one of the most provocative ever from the reclusive state. bbc.com