North Korea Claims It Has Tested Hydrogen Bomb

"The republic's first hydrogen bomb test has been successfully performed at 10:00 am on January 6, 2016, based on the strategic determination of the Workers' Party," a state television news reader said.

"With the perfect success of our historic H-bomb, we have joined the rank of advanced nuclear states," the announcer added, claiming the test was of a "miniaturised" device.

If the claim is confirmed it would mark a major, and alarming, step forward in the isolated country's nuclear development, as hydrogen bombs are generally more powerful than nuclear bombs.

A hydrogen, or thermonuclear device, uses fusion in a chain reaction that causes a much more powerful explosion than the fission blast generated by uranium or plutonium.

The country is believed to possess a handful of crude nuclear weapons, and has long pushed for an arsenal of warheads that can be mounted on a ballistic missile.

If the test is verified it will almost certainly lead to a push for new sanctions at the United Nations.

The surprise announcement came after an artificial earthquake was detected by South Korea and the United States.

In the wake of the test, South Korea's defence ministry said its armed forces would step up their monitoring of the North, while Japanese Prime Minister Shinzo Abe said it was a "serious threat" to his country and a "grave challenge" to nuclear non-proliferation efforts.

British Foreign Secretary Philip Hammond described the test as a "provocation" and a "grave breach" of UN resolutions.

South Korea's meteorological agency said earlier its neighbour was "likely" to have conducted a nuclear test that caused an earthquake close to a known testing site in the isolated country.

But the country's intelligence agency said the device tested might not have been a hydrogen bomb, the Yonhap news agency reported.

The US Defence Department said it was "looking into reports of a possible seismic event near North Korea's nuclear facilities".

The US Geological Survey reported a 5.1 magnitude quake that South Korea said was 30 miles (49km) from the Punggye-ri site where the North has conducted previous tests.

Pyongyang has so far conducted three nuclear tests - in 2006, 2009 and 2013 - with the last one also measuring 5.1 on the USGS scale.

This latest reported test was personally ordered by Kim and comes just two days before his birthday.

The North Korean leader suggested last month that Pyongyang had already developed a hydrogen bomb, a claim that was greeted with scepticism by international experts.

Most had assumed Kim was years away from obtaining a thermonuclear bomb, while assessments were divided on how far North Korea had got in mastering the technology to miniaturise a device.