US President Barack Obama said Tuesday that North Korea's "provocative" nuclear test did not make it more secure and called for "swift" and "credible" international action in response.
In a separate statement, the US intelligence community said Pyongyang had "probably conducted an underground nuclear explosion" of "approximately several kilotons" at its Punggye-ri nuclear test site.
"Analysis of the event continues," the office of the Director of National Intelligence said.
Obama vowed that Washington would remain vigilant in the face of the underground detonation by the Stalinist state and steadfast in its defense commitments to its allies in Asia.
"These provocations do not make North Korea more secure," Obama said in a written statement.
"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."
Obama complained that the test was a "highly provocative act" that -- following the North's December 12 ballistic missile launch -- undermined regional stability and violated UN Security Council resolutions.
"The danger posed by North Korea's threatening activities warrants further swift and credible action by the international community," Obama said, ahead of an emergency meeting of the UN Security Council called for later on Tuesday.
"The United States will also continue to take steps necessary to defend ourselves and our allies," Obama said.