The United States, China and Russia led a chorus of global condemnation Tuesday after North Korea defied months of stark international warnings and carried out a third nuclear test.

US President Barack Obama called for "swift and credible" action after the underground blast as even Pyongyang's top ally China voiced "firm opposition" to the nuclear test and Russia urged a halt to its "illegal actions."

The UN Security Council met in emergency session early Tuesday, after last month passing a resolution threatening "significant action" in the event of a new nuclear test.

"We certainly hope that the council will be able to send a clear message of strong condemnation," Philip Parham, Britain's deputy UN ambassador, told reporters as he entered the closed meeting in the UN's New York headquarters.

Japanese Prime Minister Shinzo Abe warned the test was a "grave threat" to his country that "cannot be tolerated," while Britain called for a "robust response" and Germany said further sanctions should be considered.

Even Iran -- under harsh UN sanctions over its own controversial nuclear program -- used the occasion to call for a nuclear weapons-free world, while defending its own atomic program, which it claims is entirely peaceful.

And nuclear-armed Pakistan, widely believed to have been the source of key technology in the 1990s that allowed North Korea to develop the bomb, said it regretted the action.

"Pakistan believes that all countries should comply with their respective international obligations," a spokesman for the foreign ministry said.

The North Korean test came just hours before Obama was to deliver his annual State of the Union address later Tuesday.

The US president called the test a "highly provocative act" that -- following the North's December 12 ballistic missile launch -- undermined regional stability and violated UN Security Council resolutions.

"North Korea's nuclear weapons and ballistic missile programs constitute a threat to US national security and to international peace and security," Obama said in a statement.

"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community," he said, adding the United States would do whatever was necessary to defend its allies.

The United States, China, Russia and the other major powers face intense pressure to act over North Korea's defiance of sanctions imposed after previous tests in 2006 and 2009.

North Korea said it staged a successful test of a "miniaturized" bomb, and the US intelligence community said Pyongyang had "probably conducted an underground nuclear explosion" of "approximately several kilotons."

The office of the Director of National Intelligence said it was continuing to analyze the event that happened at the Punggye-ri nuclear test site.

UN chief Ban Ki-moon also condemned the test as "a clear and grave violation" of the resolutions and called on North Korea to "reverse course."

China, the isolated North's closest ally providing trade and aid, expressed "strong dissatisfaction and firm opposition" when it summoned Pyongyang's ambassador.

Beijing urged its neighbor "to honor its commitment to denuclearization, and not to take any actions which might worsen the situation."

Russian Foreign Minister Sergei Lavrov said he expected the UN Security Council to agree on "an adequate response" to the test.

"North Korea should stop its nuclear program and should get back onto the non-proliferation treaty," Lavrov said. "Then it will stop its international isolation."

Rounding out the opposition of all five permanent members of the Security Council, France's President Francois Hollande condemned the blast "in the strongest terms" and said France would back "strong action" by the council.

Canadian Foreign Minister John Baird said the test showed "the North Korean regime's reckless disregard for the global will" adding it was "unconscionable" that Pyongyang "squanders limited resources" while its people starve.

In the Asia-Pacific region, nuclear-armed India called the test a "matter of deep concern," while Australia, the Philippines and Taiwan condemned the blast.

Beijing had made a special effort to try to head off the move, according to a UN diplomat who has taken part in recent consultations.

"The Chinese gave the North Koreans a strong warning against carrying out a test as it became apparent that it was imminent," said the diplomat, speaking on condition of anonymity.

"What the North Koreans have done now is a big challenge to the Chinese," the diplomat said.

Brazil meanwhile said it was deeply concerned and urged Pyongyang to comply with UN resolutions.