Obama's assurance came during his telephone talk Thursday with Japanese Prime Minister Shinzo Abe to discuss North Korea's third nuclear test and to consult on steps to respond to "this highly provocative violation of North Korea's international obligations," the White House said.
"President Obama reaffirmed that the United States remains steadfast in its defense commitments to Japan, including the extended deterrence offered by the U.S. nuclear umbrella," the White House said.
Obama and Abe "pledged to work closely together to seek significant action at the United Nations Security Council and to cooperate on measures aimed at impeding North Korea's nuclear and ballistic missile programs."
North Korea's third nuclear test Tuesday came despite the tough sanctions imposed on it by the United Nations for its similar tests in 2006 and 2009. Those sanctions were tightened further last month by the U.N. Security Council after the Communist country went ahead with its Dec. 12 long-range rocket launch, seen by other countries as a cover to test its inter-continental ballistic missile capability.
Abe is scheduled to visit Washington later this month when he will have an opportunity to hold further discussions with the U.S. President.
Japan's Kyodo News, quoting a Japanese official, reported Obama and Abe also agreed to seek a new U.N. Security Council resolution authorizing tougher sanctions on North Korea in response to its latest nuclear test.
The two also agreed Japan, the United States and South Korea should work closely in dealing with North Korean issues, the Japanese official was quoted as saying. Abe also expressed hope support from China would be forthcoming for a new resolution, the official said.