US Secretary of State Hillary Clinton spent Monday engaged in "intensive diplomacy" concerning North Korea's reported nuclear test, according to the State Department.
She had spoken by phone to her Japanese and South Korean counterparts by press time and was due to consult with Chinese and Russian leaders later in the day.
While it wasn't immediately clear what steps the US would be taking in response to the test, US President Barack Obama paused before his Memorial Day visit to Arlington Cemetery Monday morning to denounce the "blatant violation of international law."
He called the test a threat to the populations in the region and a violation of North Korea's own commitments made under multilateral negotiations - known as the six-party talks - over ending its nuclear program.
"The United States and the international community must take action in response," he declared. "North Korea will not find security and respect through threats and illegal weapons."
Obama added, "We will work with our friends and our allies to stand up to this behavior and we will redouble our efforts toward a more robust international nonproliferation regime that all countries have responsibilities to meet."