North Korea has detonated a hydrogen bomb with "perfect success", the country's state media has announced.

It claimed the device could be mounted on its newly developed intercontinental ballistic missiles, which experts have said are capable of reaching the US mainland.

Pyongyang's only major ally, China, said it strongly condemned the detonation.

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

Image: The quake was detected close to a nuclear test site in the northeast of the country

The country'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 the Japanese city of Nagasaki in World War II.

Image: An official announcement was made on North Korean state media

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 measuring 4.6 was also detected, South Korea's Yonhap news agency said.

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

Image: The authoritarian regime has not been deterred by warnings from Donald Trump

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 US President Donald Trump on the phone about the "escalating" situation.

Image: The blast was up to five times larger than the bomb dropped on Japan's Nagasaki in 1945

South Korea has called for the "strongest possible" response, including new sanctions from the UN Security Council to "completely isolate" its northern neighbour.

Seoul's national security adviser, Chung Eui-yong, said there had been talks with Washington about deploying US strategic military assets to the Korean peninsula.

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."

Sky News' Diplomatic Editor, Dominic Waghorn, said Mr Trump had to respond "one way or another, so there's a real danger here of provocation, escalation, miscalculation, leading to something really quite devastating in that part of the world".

UK and US 'short of options' to deal with N Korea

The blast came 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 where he inspected the purported device.

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".

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

What action could be taken against N Korea?

Following the release of the images, Melissa Hanham, of the Middlebury Institute for International Studies in California, said it was unknown if "this thing is full of styrofoam".

She wrote 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 North's progress towards acquiring nuclear missiles has heightened tensions on the Korean Peninsula and in Washington.

North Korea fires missile over Japan

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

It led Mr Trump to warn of unleashing "fire and fury" on the North, while Pyongyang threatened to launch missiles towards the American island 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 by the isolated communist state.