Here's how countries have reacted to the news that North Korea has likely conducted its sixth nuclear test:

• South Korea has called an emergency meeting of the National Security Council.
• Japan's Foreign Minister Taro Kono said he condemned the test in the strongest terms, calling it an unforgiveable violation of UN resolutions.
• Japanese PM Shinzo Abe had said another nuclear test would be "absolutely unacceptable".
• There doesn't appear to be any comment yet from China or the US.

Russians in the eastern city of Vladivostok are saying they felt the tremors from the North Korea.

The tremors were felt in the southwest of Primorsky Territory, Primamedia reported.

Toshiyuki Matsumori, of the Japan Meteorological Agency, is seen showing graphs of ground motion waveform data observed in Japan, following the suspected nuclear test in North Korea.

Image source, Reuters

Hydrogen bombs are the most powerful weapons ever created. They essentially work the same way as the sun.

North Korea has claimed to have tested an H-bomb before, in October last year, but that was never confirmed.

"A regular nuclear bomb is about the same as 10,000 tonnes of TNT explosives, these hydrogen bombs are at least 10 times more powerful," Paddy Regan, professor of physics at the University of Surrey, explained then.

Earlier on Sunday, North Korea's state media released this image, which it said was leader Kim Jong-un supervising a hydrogen bomb being loaded onto an intercontinental ballistic missile.

While North Korea's nuclear and weapons programmes have advanced considerably in recent years, observers have not yet confirmed that it is able to make a device small enough to fit onto one of its powerful missiles. This unverified claim is clearly challenging that scepticism.

Image source, EPA

The BBC spoke to defence expert Melissa Hanham who notes that the USGS revised upwards its initial estimate of the strength of the tremor - from 5.6 to 6.3.

If this holds, then this explosion is an order of magnitude greater than we have ever seen before from the North.

According to Reuters news agency, the Japan Meteorological Agency says tremors in North Korea were at least 10 times as powerful as the previous nuclear test detected in that country.

Japanese Prime Minister Shinzo Abe said a "forcibly conducted" nuclear test by Pyongyang would be "absolutely unacceptable," "We have to strongly protest it," he added.

Image source, AFP

"Not every seismic detection tells a horrifying story, but just rarely they pick up something special, something big, intense, and scary."

Alerts about an "artificial earthquake" have always been the world's first indication that North Korea has conducted a nuclear test.

After its previous test last year, geophysicist and disaster researcher Mika McKinnon explained the global systems that listen for tremors and can tell very quickly what has gone on.

These people in Seoul are seen watching a news broadcast on the suspected nuclear test by their neighbours to the north.

Image source, AFP

Sue Mi Terry is a former North Korea Analyst with the CIA - and was director of the Korea-Japan desk at the National Security Council, under both the George W Bush and Obama administrations.

She told BBC World News TV all eyes will now be on how President Trump reacts:

A military option against North Korea was "not on the table, no matter what people say", Sue Mi Terry added.

There are reports that the North Korean administration will make an "special announcement" at 15:00 local time (07:30 BST).

The tremors detected in North Korea were a nuclear explosion, officials in Japan say. Speaking after a meeting of the country's National Security Council, Foreign Minister Taro Kono said: "After examining the data we concluded that it was a nuclear tests."

Michael Duitsman, from the James Martin Center for Nonproliferation Studies, says North Korea wants to be seen "as a peer" on the international stage.

He added: "It wants to be seen sort of as the United States or the Soviet Union during the Cold War. They don't want to be negotiated with as a regional power, they want to be negotiated with and recognised as a global power."

Japan's Ministry of Defence said it had dispatched at least three military jets from bases in Japan to test for radiation.

A defence analyst has said the size of tremors felt in North Korea would be significant, if it was caused by a nuclear device.

Rand Corporation's Bruce Bennett said the 6.3 magnitude quake would signify "a very big weapon".

He added: "It's still not a true hydrogen bomb, but it's certainly much closer to that than anything they have ever done before."

Japanese authorities have declared that a 6.3 magnitude earthquake in North Korea was caused by a nuclear explosion.

The earthquake occurred close to a nuclear testing site. It would be North Korea's sixth nuclear test to date.

An earthquake near a nuclear test site in North Korea is suspected to have been caused by an explosion, according to South Korea officials.

Nuclear security expert Duyeon Kim said: "All of this is certainly concerning but it is not surprising. We have expected all of this to happen - it was always just a matter of time.

"Kim Jong-un's basically trying to achieve multiple objectives: achieve a technological objective of perfecting nuclear tipped missiles, achieve political objective by gaining leverage if and when negotiations ever do begin with the United States.

"And also achieving a domestic objective by showing his people the North is mighty and powerful and indestructible."

A US researcher has said that if Sunday's earthquake was caused by a nuclear blast, it would be the largest atomic test conducted by North Korea.

Dave Schmerler, of the James Martin Center for Nonproliferation Studies, said more information and analysis was needed to ascertain whether the quake was caused by a nuclear blast, but added: "This would be the largest nuclear test that North Korea has ever conducted.

"We should definitely be alarmed. Every time north Korea carries out a nuclear test isn't a good thing, it just reinforces the fact North Korea is very committed to their strategic weapons programme."