The UN Security Council ordered emergency talks Tuesday on North Korea's nuclear weapon test, which UN leader Ban Ki-moon condemned as a "deeply destabilizing" provocation.
The isolated North Korean government ignored a "strong warning" from its close ally China against staging the test, a UN diplomat said.
The 15-nation council, which called closed talks for 9:00 am New York time (1400 GMT), passed a resolution last month threatening "significant action" against Pyongyang in the event of a new nuclear test or missile launch.
The United States, China, Russia and the other major powers now face intense pressure to act over North Korea's defiance of sanctions imposed after previous tests in 2006 and 2009.
North Korea says it staged a successful test of a "miniaturized" bomb in a declaration that brought immediate global protests.
The UN secretary general condemned the underground test as "a clear and grave violation of the relevant Security Council resolutions," said his spokesman Martin Nesirky.
"It is deplorable that Pyongyang defied the strong and unequivocal call from the international community to refrain from any further provocative measures," Ban added in the comments.
Ban is "gravely concerned about the negative impact of this deeply destabilizing act on regional stability as well as the global efforts for nuclear non-proliferation."
He called on North Korea to "reverse course and work towards de-nuclearization of the Korean peninsula," and expressed confidence that the Security Council "will remain united and take appropriate action."
The United States and South Korea, the council president for February, had both called for firm measures against North Korea if it stages a nuclear test.
The South's Foreign Minister Kim Sung-Hwan is in New York for other UN Security Council meetings.
China, the isolated North's closest ally, had made a special effort to try to head off the bomb test, said 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.
China has "special means of communications" with the entourage of North Korean leader Kim Jong-Un, the envoy added.
"What the North Koreans have done now is a big challenge to the Chinese. There have been consultations in recent days, and in all likelihood China, Russia and the United States will quickly agree that tough action now has to be taken," the diplomat said.
The envoy added that sanctions may not be agreed upon on Tuesday, but the "intention" would be made clear.
It took weeks for the Security Council to agree on statements and sanctions after North Korea's nuclear tests in 2006 and 2009, and North Korea's sinking of a South Korean warship in 2010.
China has traditionally shielded its neighbor from international action, warning Western powers against any measures that could cause instability so close to its frontier.
But the Security Council ordered tougher sanctions against North Korea last month for a rocket launch it staged on 12 December, triggering a defiant pledge by Pyongyang to bolster its nuclear deterrent.
The Security Council added North Korea's state space agency, a bank, four trading companies and four individuals to its existing sanctions list.
China agreed to add to the resolution a threat of "significant action" in response to a North Korean nuclear test.