The talks come amid unanimous denunciation by the major powers of Pyongyang.
The 15-member council was expected to agree at least to a statement of condemnation at the first closed door consultations on the new proliferation crisis, diplomats said.
"I hope there will be more than a statement," said France's UN envoy Gerard Araud as he entered the meeting. "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.
The council last month threatened "significant action" against North Korea if it staged a test. The North already faces stringent UN sanctions over its weapon explosions in 2006 and 2009.
North Korea says it staged a successful test of a "miniaturized" bomb in a defiant move that brought a rare show of united protest by the Security Council's five permanent members – the United States, China, Russia, Britain and France.
China, Pyongyang's closest ally, summoned North Korea's ambassador in Beijing to lodge a protest.
International action
Russia's Foreign Minister Sergei Lavrov said he expected Security Council measures. "Such actions that are worth condemnation require an adequate response," he declared on a visit to South Africa.
US President Barack Obama called for "swift" and "credible" international action against North Korea.
Diplomats said that despite the intense pressure for sanctions, the council would likely agree a statement on Tuesday and then quickly start work on a resolution on sanctions.
The Security Council ordered sanctions against North Korea for its two earlier tests and these were toughened last month for a rocket launch the North staged on December 12.
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" if North Korea staged a new test.
China
The United States and its allies will press for swift measures, diplomats said. It took weeks for the Security Council to agree on statements and sanctions after the 2006 and 2009 tests, and North Korea's sinking of a South Korean warship in 2010.
Much focus will now be placed on how tough China is prepared to be with its neighbor, which it has traditionally shielded from international action, diplomats and experts said.
China made a special effort to try to head off the latest 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.
"What the North Koreans have done now is a big challenge to the Chinese," added the envoy.
Limited influence
"China's inability to dissuade North Korea from carrying through with this third nuclear test reveals Beijing's limited influence over Pyongyang's actions in unusually stark terms," said Suzanne DiMaggio, vice president of the Asia Society, a New York based policy forum.
"Bluntly put, North Korea's new young leader Kim Jong-Un has embarrassed China's leadership with this latest provocation."
DiMaggio said China was unlikely to impose unilateral sanctions or cut its aid to the North "but this test leaves China little choice but to support stronger international sanctions."
UN Secretary General Ban Ki-moon condemned the underground test as "a clear and grave violation" of Security Council resolutions and a "deeply destabilizing act" against regional stability, said his spokesperson. – AFP