In a statement, Obama called Pyongyang's third nuclear test in seven years a "highly provocative act" that threatens U.S. security and international peace. The reaction from the White House was significantly stronger than it was after North Korea's long-range missile test in December, when the administration only promised "appropriate action" alongside America's allies.
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community," Obama said in a statement early Tuesday. "The United States will also continue to take steps necessary to defend ourselves and our allies."
Secretary of State John Kerry spoke by telephone with the foreign ministers of South Korea and Japan after the test, as well as China, and Obama called South Korean President Lee Myung-bak to assure him the U.S. would help defend his country.
North Korea said it successfully detonated a miniaturized nuclear device at a northeastern test site Tuesday. South Korean, U.S. and Japanese seismic monitoring agencies said they detected an earthquake in North Korea with a magnitude between 4.
North Korea's official Korean Central News Agency said the test was conducted safely but with "great explosive power." The test counters the "ferocious" U.S. hostility that undermines the North's peaceful, sovereign right to launch satellites, it argued. Last month, North Korea's National Defense Commission said the United States was its prime target for a nuclear test and long-range rocket launches.
"These provocations do not make North Korea more secure," Obama said. "Far from achieving its stated goal of becoming a strong and prosperous nation, North Korea has instead increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction and their means of delivery."
The Obama administration's options for a response are limited—even though it is committed to protecting America's key Asian allies, South Korea and Japan.
The U.S. already maintains severe unilateral sanctions against North Korea, and commerce between the two countries is nearly nonexistent. Tougher global sanctions are dependent on the participation of China, Pyongyang's primary trading partner, but Beijing has resisted measures that would cut off North Korea's economy completely.
At the U.N., the Security Council opened an emergency meeting Tuesday and condemned North Korea's action. But any new, binding international sanctions will now have to be worked out.
More forceful U.S. consequences, in the form of a military response, are highly unlikely even though the United States remains technically at war with the notoriously unpredictable North Koreans, whose opaque leadership has confounded successive American administrations. Only the 1953 armistice ending the Korean War keeps the U.S. and the North from hostilities, and some 28,500 U.S. troops in South Korea to deter potential aggression.
But with the South Korean capital of Seoul just 40 miles south of the nuclear-armed North's border and its million-man army, the risks involved in any military action are great. And just raising the rhetoric can even serve as a reward to North Korea's attention-seeking government, which starves its citizens while seeking to leverage any military advance it makes into much-needed aid.
The test occurred hours before Obama was to deliver his prime-time address to the nation. While the bulk of the speech will focus on the economy and job creation, the administration had sought to emphasize that it wants to work with Russia on further reducing the amount of deployable nuclear weapons each power maintains, building on the U.S.-Russian New START accord that took effect in 2011.
It's unclear how North Korea's test would affect the planning. Tommy Vietor, a spokesman for the White House's National Security Council, said Obama would make the case Tuesday evening that the "only way North Korea will rejoin the world community is if they stop these threats and live up to their international obligations."
State Department spokeswoman Victoria Nuland added that North Korea's test was unrelated to the president's position that the U.S. has "more nuclear weapons than we need."
"We're obviously talking about apples and oranges in the context of the U.S. having almost 1,700 nuclear weapons and we are working to prevent (North Korea) from having the effective ability to launch a far smaller amount," she said.
Defense Secretary Leon Panetta, in remarks Tuesday morning to Pentagon workers gathered in the building's courtyard, said the U.S. is going to have to continue to deal with rogue states like North Korea.
"We just saw what North Korea has done in these last few weeks, a missile test and now a nuclear test," he said. "They represent a serious threat to the United States of America, and we've got to be prepared to deal with that. "
The chairman of the House Foreign Affairs Committee, California Republican Ed Royce, called for even tougher sanctions and demanded that the administration "replace its failed North Korea policy."
"Otherwise, the grave North Korean threat to the region and the United States will only grow," Royce said.
And House Intelligence Committee chairman Mike Rogers, R-Mich., added that the U.S. must take a new approach to dealing with North Korea. "The key to stemming North Korea's cycle of provocation is to seriously engage the Chinese in exercising leverage over their neighbor," he said.
On Tuesday, China expressed firm opposition to the test but called for a calm response by all sides.
Associated Press writer Lolita C. Baldor and AP Intelligence Writer Kimberly Dozier contributed to this report.