The UN Security Council on Tuesday "strongly condemned" North Korea's third nuclear test amid US demands that it face new sanctions to halt its weapons development.
A unanimously agreed Security Council statement said that because of the "gravity" of North Korea's defiant gesture, the 15-member council "will begin work immediately on appropriate measures".
"The members of the Security Council strongly condemned this test, which is a grave violation" of three council resolutions, said the statement, read to reporters by South Korea's Foreign Minister Kim Sung-Hwan after emergency consultations.
"There continues to exist a clear threat to international peace and security," added the statement, which highlighted a threat made by the council in January to take "significant action" if North Korea staged a new nuclear test.
"In line with this commitment and the gravity of this violation, the members of the Security Council will begin work immediately on appropriate measures in a Security Council resolution."
The statement made no direct reference to sanctions but demands for new measures were quickly made by the United States and its allies.
"To address the persisting danger posed by North Korea's threatening activities, the UN Security Council must and will deliver a swift, credible and strong response," US ambassador Susan Rice told reporters.
Pyongyang's actions "will not be tolerated and they will be met with North Korea's increasing isolation and pressure under United Nations sanctions," she said.
The Council must pass a resolution that "further impedes the growth of DPRK's nuclear weapons and ballistic missile programs and its ability to engage in proliferation activities," Rice said, using the North's official name, the Democratic People's Republic of Korea.