The apparent blast caused a 5.3-magnitude seismic event
Foreign Affairs Minister Charlie Flanagan has expressed "grave concern" about reports North Korea tested a nuclear explosive device.
The country says it conducted a "nuclear warhead explosion" designed to counter alleged hostility from the United States.
Its fifth and seemingly largest nuclear test caused a 5.3-magnitude seismic event and was described by South Korea as "maniacal recklessness".
A state TV announcer in the North confirmed the detonation at the Pyunggye-ri nuclear test site.
She said: "Our...party sent a congratulatory message to our nuclear scientists...for conducting the successful nuclear warhead explosion test."
Pyongyang said the success of the test - which drew immediate international condemnation - meant it could produce "at will a variety of smaller, lighter and diversified nuclear warheads of higher strike power".
It claimed there was no radioactive leakage or adverse environmental impact - and neighbouring Russia has reported no increase in radiation on its eastern border.
China, North Korea's economic lifeline and only major ally, said it "firmly opposed" the test and would monitor radiation along its northeastern border.
Minister Flangan said: "I am gravely concerned at the latest reports that North Korea has once again tested a nuclear explosive device. This constitutes the second reported test this year, and represents a real threat to peace and security on the Korean Peninsula, and the entire North East Asia region.
"The reported test is the latest in a series of provocative actions undertaken by North Korea this year, and is a flagrant violation of its international obligations not to produce or test nuclear weapons.
"It shows a blatant disregard for recent efforts of the international community to encourage North Korea to cease such actions."
US President Barack Obama was briefed on Air Force One as he returned from a trip to Asia - and warned of "serious consequences" for the secretive state.
He called the leaders of South Korea and Japan to reiterate his country's "unbreakable" commitment to protecting allies in the region.
"The president indicated he would continue to consult our allies and partners in the days ahead to ensure provocative actions from North Korea are met with serious consequences," White House press secretary Josh Earnest said.
France also criticised North Korea's actions, while 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.
South Korean President accused Kim Jong-Un of "maniacal recklessness", with the country's military estimating 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.
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.