Clinton also underscored the firmness of the U.S. treaty commitment to defend South Korea and Japan, U.S. allies in easy range of North Korean missiles.
Clinton's stern statement came after North Korea threatened military attacks against U.S. and South Korean warships and called Seoul's decision to join an international program to intercept ships suspected of aiding nuclear proliferation tantamount to a declaration of war.
"North Korea has made a choice" to violate U.N. Security Council resolutions, ignore international warnings and abrogate commitments made during six-nation nuclear disarmament talks, Clinton said.
"There are consequences to such actions," she said, referring to discussions in the United Nations meant to punish North Korea for its nuclear and missile tests.
Clinton did not provide specifics, saying only that the intent of diplomats was to "try to rein in the North Koreans" and get them to fulfill commitments made in the nuclear talks.
Clinton said she was pleased by a unified international condemnation of North Korea that included Russia and China, North Korea's only major ally and the host of the currently stalled disarmament talks.
The success of any new sanctions would depend on how aggressively China implements them.
Despite her tough words, Clinton held out hope that North Korea would return to six-nation disarmament talks and that "we can begin once again to see results from working with the North Koreans toward denuclearization that will benefit, we believe, the people of North Korea, the region and the world."
The North Korean army called South Korea's actions a violation of the armistice the two Koreas signed in 1953 to end their three-year war, and said it would no longer honor the treaty.
At the White House, spokesman Robert Gibbs played down North Korea's angry rhetoric. He said Wednesday that North Korea's threats against South Korea will not give it the attention Pyongyang wants and will only add to its isolation.
Gibbs said North Korea has threatened to end the armistice many times in past decades but the peace has held.
The Pentagon was still testing and analyzing particle matter taken from clouds in the region to confirm that the North's detonation was, indeed, a nuclear explosion. A senior Pentagon official said U.S. military jets were to take a second sampling later this week.
Associated Press writers Lara Jakes and Philip Elliott contributed to this report.
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