The U.N. Security Council is pledging further action against North Korea, calling today's nuclear test a "clear threat to international peace and security."

North Korea had earlier conducted tests in the weeks after it launched rockets in 2006 and 2009 -- and this third test followed a rocket launch in December. The council says it's a "grave violation" of U.N. resolutions aimed at limiting the North's nuclear program.

North Korea is calling the atomic test its "first response" to what it describes as U.S. threats. It says it will continue with "second and third measures of greater intensity" if Washington maintains its hostility.

The underground test, which set off powerful seismic waves, drew condemnation from around the world. Even North Korea's only major ally, China, is denouncing the test and demanding that North Korea return to "dialogue and negotiation." But it remains to be seen whether China will sign on to any new, binding global sanctions.

President Barack Obama, in a statement, says North Korea is further isolating itself and putting its people further into poverty.