U.S. President Barack Obama said Tuesday that North Korea's announcement of another nuclear test is "highly provocative act" and pledged all necessary actions to defend his country and its allies, Yonhap News said in a report.
In a statement released nearly four hours after the first report of North Korea's apparent nuclear test, Obama said it violates the isolated communist nation's obligations under numerous United Nations Security Council resolutions.
It also "contravenes its commitments under the September 19, 2005 Joint Statement of the Six-Party Talks, and increases the risk of proliferation," he said.
"The United States remains vigilant in the face of North Korean provocations and steadfast in our defense commitments to allies in the region," Obama said. "The United States will also continue to take steps necessary to defend ourselves and our allies."
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community," he added.
Obama's response followed a statement by the U.S. Office of the Director of National Intelligence (DNI) that it's evaluating all information related with an unusual seismic activity in North Korea.
North Korea announced through its official media that it has successfully conducted an underground nuclear test.
If confirmed, it would be Pyongyang's third known nuclear test following the two previous ones in 2006 and 2009.
The U.N. Security Council is scheduled to hold an emergency session Tuesday morning in New York to discuss the issue.
North Korea's nuclear test came on the eve of Obama's State of the Union speech, the first of his second term.
Obama plans to re-emphasize his vision of a nuclear-free world in his speech at Capitol Hill, according to administration officials.
Pyongyang's move is seen as a heavy blow to Obama's initiative for nonproliferation and arms control.
"As North Korea is almost certain to have carried out another nuclear test, chances are high that President Obama will talk about the North Korea issue in his State of the Union address," a diplomatic observer said.
For many, North Korea's third nuclear test came as no surprise since it had publicly threatened to do so while carrying out activities at its nuclear test area.
One key question is whether North Korea used uranium. In its two previous tests it detonated plutonium-made bombs.
A uranium-based nuclear device poses a more serious threat to global security because it is very hard to trace.
It is expected to take several days to gather air samples to confirm whether uranium was used.
U.S. officials have delivered a consistent message that Washington would deal with a nuclear test in a stern and resolute manner.
"We don't take anything off the table. We never do," State Department spokeswoman Victoria Nuland said last week during a press briefing.
The U.S. and its allies will likely focus first on hammering out a unified response in the U.N. Security Council, followed by implementing their own punitive steps.
In a new resolution against North Korea for its December long-range rocket launch, the council expressed "its determination to take significant action in the event of a further DPRK (North Korea) launch or nuclear test."
If Pyongyang actually conducted a nuclear test, experts say, it would test not only Obama's patience but also the policy of China's leader-in-waiting Xi Jinping.
"We will soon learn the Chinese definition of 'serious action,'" said Gordon Flake, the executive director of the Maureen and Mike Mansfield Foundation.
Nicholas Eberstadt, a researcher at the American Enterprise Institute, said North Korea's continued nuclear tests and ballistic rocket launches are not a surprise.
"For North Korean leadership, doing so makes urgent sense -- for this is an engineering matter of the highest priority for that state," he said. "Pyongyang will continue to test nukes and missiles until it is confident it has perfected its capability for targeting the U.S. homeland."