The U.N. Security Council will aim for swift condemnation of North Korea's nuclear test at an emergency meeting Wednesday and follow up with more sanctions against the country, the British ambassador said.

"The Security Council needs to be clear in its condemnation and robust in its response," Matthew Rycroft told reporters before the meeting.

The council was meeting hours after North Korea announced its first hydrogen bomb test, which would mark a major advance for its still-limited nuclear arsenal. The announcement was met with skepticism, however, with South Korea's spy agency saying the estimated explosive yield from the explosion was much smaller than what even a failed H-bomb detonation would produce.

The Security Council has imposed four sets of sanctions against North Korea since the country's first nuclear test in 2006. The sanctions are aimed at reining in the North's nuclear and missile development, but Pyongyang has ignored them and moved ahead with programs to modernize its ballistic missiles and nuclear weapons.

Rycroft said the test is another example of Pyongyang's "reckless challenge to international norms of behavior and the authority of the U.N. Security Council." He condemned the North's "provocation which endangers international peace and security."

U.N. Secretary-General Ban Ki-moon called North Korea's announcement "profoundly destabilizing for regional security" and a violation of Security Council resolutions. Ban demanded that Pyongyang cease any further nuclear activities.

The council last approved sanctions against North Korea three weeks after Pyongyang's third nuclear test on Feb. 12, 2013.

U.N. diplomats, speaking on condition of anonymity because talks have been private, said a new resolution would aim to add more people to the sanctions list, including those linked to a key procurement company, and limit the travel of senior North Korean officials.

How tough the new sanctions will be depends largely on China.

Russian Ambassador Vitaly Churkin said what is needed is "Cool heads ... Proportionate response." New Zealand Ambassador Gerard van Bohemen said he hoped there would be new sanctions.