North Korea says it has carried out its sixth nuclear test, this time using a newly built hydrogen bomb. The blast is the most powerful to date and caused a significant earthquake.

Foreign Minister Gerry Brownlee has condemned Sunday's nuclear test by North Korea as "utterly deplorable".

North Korea has confirmed it successfully tested a hydrogen bomb meant for an intercontinental ballistic missile.

The move is a direct challenge to US President Donald Trump, who hours earlier had talked by phone with Japanese Prime Minister Shinzo Abe about the "escalating" nuclear crisis in the region.

Previous earthquakes near North Korea have been the result of nuclear tests.

​Brownlee said in a statement the test was "a highly dangerous affront to the entire international community".

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KEVIN LAMARQUE/REUTERS US President Donald Trump is in the midst of dealing with the aftermath of Hurricane Harvey. Now he also may have to worry about North Korea conducting its sixth nuclear test, the first since he took office.

"North Korea has again demonstrated its complete disregard for its obligations under United Nations Security Council resolutions, and for international norms against nuclear testing.

"North Korea has a choice. It can continue on this path, inviting further pressure and isolation and entrenching poverty and misery for its own people. Or it can choose membership of the international community and the security and prosperity that comes with it. Only North Korea can make this decision."

Brownlee said New Zealand would consider what measures it could take in response to the situation.

Sunday's test is the country's sixth and most powerful nuclear test, thought it is the first since Trump took office.

The hydrogen bomb test ordered by leader Kim Jong Un was a "perfect success" and was a "meaningful" step in completing the country's nuclear weapons programme, state television said.

China's Foreign Ministry on Sunday urged North Korea to stop its "wrong" actions, after Pyongyang said it successfully tested an advanced hydrogen bomb.

The ministry said in a statement on its website that China resolutely opposed and strongly condemned North Korea's actions, and urged the country to respect UN Security Council resolutions.

Japanese and South Korean meteorological officials said an earthquake detected near the North's test site - measured by the US Geological Survey (USGS) at magnitude 6.3 - was around 10 times more powerful than previous detonations.

Japan immediately raised the prospect of further sanctions against the isolated North, with Chief Cabinet Secretary Yoshihide Suga saying that curbs on its oil trade would be on the table.

A US official who studies North Korea's military and politics said that seismic data on the tremors was being analysed, although the location suggested another nuclear test.

The official, who spoke on the condition of anonymity, said it was too early to determine if a test, if there was one, supported the North's claim that has succeeded in developing a thermonuclear weapon, "much less one that could be mounted on an ICBM and re-enter Earth's atmosphere without burning up".

The hydrogen bomb report by North Korea's official KCNA news agency comes amid heightened regional tension following Pyongyang's two tests of intercontinental ballistic missiles (ICBM) in July that potentially could fly about 10,000 km, putting many parts of the mainland United States within range.

Under third-generation leader Kim Jong Un, North Korea has been pursuing a nuclear device small and light enough to fit on a long-range ballistic missile, without affecting its range and making it capable of surviving re-entry into the Earth's atmosphere.

One expert said the size of Sunday's detonation meant it was possible it could be a hydrogen bomb test.

"The power is 10 or 20 times or even more than previous ones," Said Kune Y. Suh, a nuclear engineering professor at Seoul National University. "That scale is to the level where anyone can say a hydrogen bomb test."

HOURGLASS-SHAPED DEVICE

Kim Jong Un, who visited the country's nuclear weapons institute, "watched an H-bomb to be loaded into new ICBM" and "set forth tasks to be fulfilled in the research into nukes," KCNA said.

Pictures released by the agency showed Kim inspecting a silver-coloured, hourglass-shaped warhead in the visit accompanied by nuclear scientists.

The shape shows a marked difference from pictures of the ball-shaped device North Korea released in March last year, and appears to indicate the appearance of a two-stage thermonuclear weapon, or a hydrogen bomb, said Lee Choon-geun, senior research fellow at state-run Science and Technology Policy Institute.

"The pictures show a more complete form of a possible hydrogen bomb, with a primary fission bomb and a secondary fusion stage connected together in an hourglass shape," Lee said.

Tensions on the Korean peninsula have been high since last month when North Korea threatened to launch missiles into the sea near the strategically located US Pacific territory of Guam after Trump said Pyongyang would face "fire and fury" if it threatened the United States.

North Korea further raised regional tensions on Tuesday by launching an intermediate-range ballistic missile over Japan, drawing international condemnation.

Trump and Abe spoke by phone and said that in face of an "escalating" situation with North Korea that close cooperation between their countries and with South Korea was needed, Abe told reporters.

The United States has repeatedly urged China, the North's sole major ally, to do more to rein in its neighbour.

Impoverished North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and its main ally, the United States.

- Stuff, agencies