Two missiles - one ground-to-air, the other ground-to-ship - with a range of about 80 miles (130 kilometers) were test-fired from an east coast launchpad, South Korea's Yonhap news agency reported, citing an unidentified government official.
Pyongyang also warned ships to stay away from waters off its western coast this week, a sign it may be gearing up for more missile tests, South Korea's coast guard said.
North Korea is "trying to test whether they can intimidate the international community" with its nuclear and missile activity, said Susan Rice, U.S. ambassador to the United Nations.
"But we are united, North Korea is isolated and pressure on North Korea will increase," Rice said. On Monday, President Barack Obama assailed Pyongyang, accusing it of engaging in "reckless" actions that have endangered the region, and the North accused Washington of hostility.
Wall Street was lower in early trading as North Korea's actions kept investors on edge.
North Korea appeared to be displaying its might following its underground atomic test that the U.N. Security Council condemned as a "clear violation" of a 2006 resolution banning the regime from developing its nuclear program.
France called for new sanctions, while the U.S. and Japan pushed for strong action against North Korea for testing a bomb that Russian officials said was comparable in power to those dropped on the Japanese cities of Hiroshima and Nagasaki in World War II.
China said it "resolutely opposed" North Korea's test and urged Pyongyang to return to talks on ending its atomic programs.
Russia, once a key backer of North Korea, condemned the test. Moscow's U.N. Ambassador Vitaly Churkin, the current Security Council president, said the 15-member council would begin work "quickly" on a new resolution.
But many questioned whether new punishment would have any effect on a nation already penalized by numerous sanctions and clearly dismissive of the Security Council's jurisdiction.
"I agree that the North Koreans are recalcitrant and very difficult to hold to any agreement that they sign up to," Britain's ambassador to the U.N., John Sawers, told the British Broadcasting Corp. "But there is a limited range of options here."
U.N. Secretary-General Ban Ki-moon said he felt "frustrated by the lack of progress in the denuclearization process" and said North Korea's only viable option was to return to the six-party talks on disarmament, and continue exchanges and cooperation with South Korea.
Ban, on a visit to Finland, declined to comment on possible further sanctions.
"I leave it to the Security Council members what measures they should take," said Ban, a South Korean who once participated in international negotiations aimed at dismantling North Korea's nuclear program.
South Korea said it would join a maritime web of more than 90 nations that intercept ships suspected of spreading weapons of mass destruction - a move North Korea warned would constitute an act of war.
North Korea's nuclear test raises worries that it could act as a facilitator of the atomic ambitions of other nations and potentially even terrorists.
Its test of a long-range missile in July 2006 and its first nuclear test in October 2006 drew stiff sanctions from the Security Council and orders to refrain from engaging in ballistic missile-related activity and to stop developing its nuclear program.
South Korean spy chief Won Sei-hoon had told lawmakers earlier Tuesday that a missile test was likely, according to the office of Park Young-sun, a legislator who attended the closed briefing.
Yonhap reported that North Korea was preparing to launch a third missile from a west coast site, again citing an unidentified official. It also reported that three missile tests were conducted Monday.
North Korea had threatened in recent weeks to carry out a nuclear test and fire long-range missiles unless the Security Council apologized for condemning Pyongyang's April 5 launch of a rocket the U.S., Japan and other nations called a test of its long-range missile technology. The North has said it put a satellite into orbit as part of its peaceful space development program.
Monday's nuclear test appeared to catch the world by surprise, but Won told lawmakers that Beijing and Washington knew Pyongyang was planning a test some 20-25 minutes before it was carried out, said Choi Kyu-ha, an aide to lawmaker Park.
Won said Pyongyang warned it would test the bomb unless the head of the Security Council offered an immediate apology. Russia said the test went off at 9:54 a.m. local time (0054 GMT Monday, 8:54 p.m. EDT Sunday). Won confirmed that two short-range missile tests from an east coast launch pad followed.
North Korea's neighbors and their allies scrambled to galvanize support for strong, united response to Pyongyang's nuclear belligerence.
Obama and South Korean President Lee Myung-bak "agreed that the test was a reckless violation of international law that compels action in response," the White House said in a statement after the leaders spoke by telephone. They also vowed to "seek and support a strong United Nations Security Council resolution with concrete measures to curtail North Korea's nuclear and missile activities."
Obama also spoke with Japanese Prime Minister Taro Aso, the White House said, with the leaders agreeing to step up coordination with South Korea, China and Russia. Obama reiterated the U.S. commitment to defend both South Korea and Japan, U.S. and South Korean officials said.
North Korea responded by accusing the U.S. of hostility, and said its army and people were ready to defeat any American invasion.
"The current U.S. administration is following in the footsteps of the previous Bush administration's reckless policy of militarily stifling North Korea," the North's main Rodong Sinmun newspaper said in commentary carried by the country's official Korean Central News Agency.
In Japan, the lower house of parliament quickly passed an unanimous resolution condemning the test and demanding that North Korea give up its nuclear program, a house spokeswoman said.
"This reckless act, along with the previous missile launch, threatened peace and stability in the region, including Japan," the resolution said.
"North Korea's repeated nuclear tests posed a grave challenge to international nuclear nonproliferation," it said. "Japan, the only nation to suffer atomic attacks, cannot tolerate this." Japan is considering tightening sanctions against North Korea, the statement said.
Russia called the test a "serious blow" to efforts to stop the spread of nuclear weapons and suspended a Russia-North Korean intergovernmental trade and economic commission, apparently in response to the test. The slap on the wrist was a telling indication that Moscow, once a key backer of North Korea, was unhappy with Pyongyang.
Seoul reacted to the nuclear test by signing on to the U.S.-led Proliferation Security Initiative, joining 94 nations seeking to intercept ships suspected of carrying nuclear, chemical and biological weapons, materials to make them, or missiles to deliver them.
North Korea for years has warned the South against joining the blockade. The Rodong Sinmun last week said South Korea's participation would be "nothing but a gambit to conceal their belligerence and justify a new northward invasion scheme."
Joining the PSI would end in Seoul's "self-destruction" it said.
In Beijing, the defense chiefs of South Korea and China held a security meeting Tuesday, and they were expected to discuss ways to respond to the nuclear test, Yonhap quoted a South Korean official as saying.
Associated Press writers Kwang-tae Kim and Jean H. Lee in Seoul, Shino Yuasa in Tokyo, Matti Huuhtanen in Helsinki, Finland, and Mike Eckel in Moscow contributed to this report.
Copyright 2009 The Associated Press.
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