UPDATED 6.40PM:  Foreign Minister Murray McCully has described North Korea's reported nuclear test today as "highly provocative" and a serious threat to regional security.

McCully has issued a statement saying as President of the Security Council, New Zealand will convene a meeting of the Security Council soon to discuss the test.

"This is the fifth nuclear test conducted by North Korea, and it directly defies the UN Security Council which has demanded that North Korea stop nuclear testing.

"This latest nuclear test is highly provocative and deeply concerning, especially as it comes on the back of a series of ballistic missile launches by North Korea recently."

It was first detected by a European monitoring agency which reported a 5.0 magnitude earthquake near North Korea's northeastern nuclear test site. 

It has since been confirmed by North Korea as a nuclear test. 

US, European and Chinese agencies said the tremor was recorded at around 9.30am (local time) on Friday. 

A South Korea government official and the meteorological agency said it was a suspected nuclear test, and Seoul called an emergency meeting of its National Security Council.

"Today at around 9:30am, we detected an artificial earthquake wave at magnitude 5.0 in the area of North Korea's Punggye-ri and we are analysing whether it is a nuclear test," South Korea's Joint Chiefs of Staff said in a statement, referring to the North's nuclear test site.

The China Earthquake Networks Center also said the tremor was a suspected explosion. China's environment ministry began emergency radiation monitoring along its borders with North Korea in northeast China, state television reported.

Japanese Prime Minister Shinzo Abe said such a nuclear test could not be tolerated. Japan's foreign minister lodged a protest and Tokyo sent two military jets to begin measuring for radiation.

Jeffrey Lewis of the California-based Middlebury Institute of International Studies said the highest estimates of seismic magnitude suggested this was the most powerful nuclear test conducted by North Korea so far.

He said the seismic magnitude and surface level indicated a blast with a 20- to 30-kilotonne yield. If confirmed, he said this would make it larger than the nuclear bomb dropped on the Japanese city of Hiroshima in the second World War and potentially bigger than that dropped on Nagasaki soon after.

"That's the largest DPRK test to date, 20-30kt, at least. Not a happy day," Lewis told Reuters, using the North's official title of the Democratic People's Republic of Korea," said Mr Lewis.

North Korean state media have reported it was of a "newly developed nuclear warhead" and that it was now capable of mounting a nuclear device on ballistic rockets.

The nuclear test comes just days after the Security Council met behind closed doors to discuss North Korea's ballistic missile testing.

Labour's Foreign Affairs spokesman David Shearer said the testing showed a "dangerous escalation" in North Korea's capabilities. He said New Zealand should take a leading role on a strategy to deal with it.

"New Zealand has consistently been a leader in nuclear non-proliferation and disarmament, most notably with the implementation of our nuclear-free policy.

"In the past eight years however, our efforts have been lacklustre and unfocused.

"It's time for our Government to re-exert our leadership in the area of nuclear disarmament as a matter of urgency." 

UN resolutions bar North Korea from any use of ballistic missile technology, but Pyongyang has carried out several launches following its fourth nuclear test in January.

North Korea has been hit by five sets of UN sanctions since it first tested a nuclear device in 2006.

The latest took place just after many world leaders, including US President Barack Obama and Russia's Prime Minister Dmitry Medvedev, wound up talks in Laos after the East Asia Summit.

Prime Minister John Key was at that summit and is now in the Federated States of Micronesia for the Pacific Islands Forum.

He will head to New York later this month to attend the UN General Assembly and Security Council meetings as part of New Zealand's month as President.​