The U.S. president promised Wednesday to defend South Korea and Japan from any attack by North Korean amid escalating threat from the reclusive state.
Mr. Obama telephoned the leaders of both countries to reiterate long-standing U.S. security guarantees.
Both leaders agreed that North Korea's decision to nuclear test was a reckless violation of international law and they would seek concrete measures to curtail North Korea's nuclear and missile activities, a statement from the U.S. State Department said.
U.S. Secretary of State Hillary Clinton also stressed Wednesday that the U.S. commitments to defend South Korea and Japan from attacks by North Korea.
I want to underscore the commitments the United States has and intends always to honor for the defense of South Korea and Japan, Clinton said. That is part of our alliance commitment that we take very seriously.
North Korea was provoked Wednesday by the South's decision to join a U.S.-led International Proliferation Security Initiative (IPS).
The Proliferation Security Initiative (PSI), set up by the Bush administration in 2003 and now joined by 90 countries, is aimed at stopping and searching shipping suspected of carrying weapons of mass destruction or their related components.
North Korean said any act against curbing their ships would be considered act of war and it would immediately respond with a powerful military strike.
The UN Security Council has issued a unanimous condemnation of Pyongyang and is now preparing to start work on a new range of sanctions against North Korea, a process which diplomats said could take up to two weeks.
China and Russia have joined the condemnation of North Korea, however both nations have argued in the past that further sanctions and isolation of Pyongyang could be counter-productive in efforts to re-start the stalled Six Party talks on nuclear disarmament.