Story Highlihts
• U.S. pushes for sanctions against North Korea at U.N. emergency meeting
• North Korea claims successful nuclear weapons test, raising arms race fears
• U.S. trying to confirm authenticity of claim
• Bush: North Korea fully accountable for consequences of transferring nukes
WASHINGTON (CNN) – President Bush on Monday said North Korea's claim that it has tested a nuclear weapon is a threat to international peace and said the world "will respond."
"The transfer of nuclear weapons to states or non-state entities would be considered a grave threat to the United States," Bush said. "And we would hold North Korea fully accountable to the consequences of such action."
Bush said the U.S. was still trying to verify North Korea's claims that it had tested a nuclear weapon on Monday.(Watch Bush on why it poses such a threat — 2:34 
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He said he'd spoken with the leaders of South Korea, Russia, China and Japan.
"All of us agreed that the proclaimed actions taken by North Korea are unacceptable and deserve an immediate response by the United Nations Security Council," Bush said. (Transcript)
At an emergency meeting of the U.N. Security Council on Monday, the United States, China, Japan, South Korea, Russia and the European Union quickly condemned North Korea's claim. (World reaction)
Senior U.S. officials said the United States will push for a Security Council resolution under Chapter 7 of the organization's charter which deals with "threats to the peace" and "acts of aggression."
The officials believe the United States has "substantial" support for "strong" sanctions. (Time.com: What can the U.S. do?
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The United States and its allies have been urging North Korea to re-join six-party talks aimed at persuading the reclusive Communist nation to abandon its nuclear arms program.
China, a close ally of North Korea, denounced the claimed test as "brazen," and South Korea said it would respond "sternly" to a move that experts said raised fears of nuclear terrorism and a regional arms race.
North Korea's ambassador to the U.N. Pak Kil-yon said the council should "congratulate" North Korea's scientists and researchers "instead of [issuing] such notorious, useless and reckless resolutions" against North Korea.
Pak said the test was "very, very successful" and will contribute "to the maintenance and guarantee of peace and security in the peninsula and the region." (Text of North Korean statement)
When asked if North Korea plans to conduct further tests, Pak told reporters "that will be enough, you don't think so?"
The announcement was made on the same day as South Korean Foreign Minister Ban Ki-Moon was formally nominated to become the next U.N. secretary-general, following Kofi Annan. (Full story)
Nuclear Nations
North Korea claimed Monday to have completed a successful underground nuclear test. If the country's claims are confirmed, North Korea would join only a handful of nations known to possess nuclear weapons.