South Korean Foreign Minister Kim Sung-hwan, repeating a statement to reporters outside council chambers, said the test violated council resolutions, and "there continues to exist a clear threat to international peace and security."
Kim said council members will start work "on appropriate measures in a Security Council resolution" and that "North Korea will be held responsible for any consequences of this provocative act." The council is chaired this month by South Korea.
A press statement approved by all 15 council members at an emergency meeting hours after the latest underground test called the atomic blast a "grave violation" of three U.N. resolutions banning the North from conducting nuclear or missile tests.
North Korea conducted nuclear weapons tests weeks after rocket launches in 2006 and 2009, and this third test followed a rocket launch in December. The Security Council pointed out that in a resolution it approved unanimously last month, which strengthened sanctions in response to the December missile test, members promised to take "significant action" in the event of 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 council said.