North Korea says it has conducted a "nuclear warhead explosion" designed to counter alleged hostility from the United States.

The country's fifth and seemingly largest nuclear test caused a 5.3-magnitude seismic event and was described by South Korea as "maniacal recklessness".

It has sparked calls from Britain, France and the US for new sanctions, with the UN Security Council saying these will be considered as part of "significant measures" against Pyongyang.

North Korea said the success at the Pyunggye-ri nuclear test site meant it could produce "at will a variety of smaller, lighter and diversified nuclear warheads of higher strike power".

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It claimed there was no radioactive leakage or adverse environmental impact - and neighbouring Russia has reported no increase in radiation on its eastern border.

Image: A state TV announcer confirms the nuclear test

China, North Korea's economic lifeline and only major ally, said it "firmly opposed" the test and would monitor radiation along its northeastern border.

US President Barack Obama directly condemned the nuclear test, saying the US will never accept North Korea as a nuclear state and warned of "serious consequences" for the secretive country.

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He called the leaders of South Korea and Japan to reiterate his country's "unbreakable" commitment to protecting allies in the region.

Germany and Russia also criticised North Korea's actions, while British Foreign Secretary Boris Johnson said "we all want to condemn strongly this kind of needless provocation".

Nearby countries have reacted furiously to the detonation, which comes on North Korea's national day.

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South Korea's military estimated the explosive power of the detonation at 10 kilotonnes - the equivalent of 10,000 tonnes of TNT.

Japan's Prime Minister has called the test "absolutely unacceptable"..

Jeffrey Lewis, a US analyst, said it appeared to be North Korea's largest test to date and claimed it boasted an explosive power of up to 30,000 tonnes of TNT - five times more than Pyongyang's last nuclear test in January.

Image: The 5.3-magnitude tremors were reported near the Pyunggye-ri nuclear test site

If this was the case, it would be larger than the bomb dropped by the US on Hiroshima in World War Two.

North Korea has also performed a series of unsuccessful missile tests in recent months which are part of its push for a nuclear-armed missile that could one day reach the American mainland.

The impoverished country has been subject to fresh sanctions after violating UN Security Council resolutions.

In July, satellite images appeared to show high levels of activity at the country's nuclear test site - and speculation has been growing that Kim Jong-Un was planning a fifth test after the US blacklisted him for human rights abuses.

Japan's Nikkei share average fell into negative territory upon news of the "seismic event".