Obama vows nuclear deterrence for S. Korea

U.S. President Barack Obama and his South Korean counterpart, President Lee Myung-bak, on Tuesday pledged unswerving unity in coping with North Korea's nuclear and ballistic missile threats.

In a phone conversation with Lee, Obama also agreed to work together on several measures to punish Pyongyang for its continued provocative acts and to curb its drive for weapons of mass destruction, according to the White House.

"They agreed to work closely together, including at the United Nations Security Council, to seek a range of measures aimed at impeding North Korea’s nuclear and ballistic missile programs and reducing the risk of proliferation," the White House said in a press release.

"President Obama unequivocally reaffirmed that the United States remains steadfast in its defense commitments to the Republic of Korea, including the extended deterrence offered by the U.S. nuclear umbrella," it added.

The phone talks between Obama and Lee came as the U.N. Security Council began discussions about a response to North Korea's nuclear test that took place a day earlier.

The council had an one-hour emergency session in New York, in which its members strongly condemned Pyongyang's behavior.

"To address the persistent danger posed by North Korea’s threatening activities, the U.N. Security Council must and will deliver a swift, credible, and strong response by way of a Security Council resolution that further impedes the growth of DPRK’s nuclear weapons and ballistic missile programs and its ability to engage in proliferation activities," U.S. Ambassador Susan Rice told reporters after the meeting.

In the days ahead, she added, the U.S. will consult closely with other Council members and other U.N. member states to pursue appropriate further action.

In Washington, the State Department said the U.S. is considering a broad range of options.

"It's fair to say that we are looking at the full suite of options to try to get the DPRK (North Korea) to change course," department spokeswoman Victoria Nuland said at a daily press briefing.

As the U.S. and its allies seek a tough and swift response by the council, an important question is the role China will play.

Nuland acknowledged the importance of Beijing, a veto-wielding member of the council.

"We've all said for quite some time that the Chinese have the most influence within the six-party group. That's obvious given their well-intermeshed economic relationship with the DPRK," Nuland said.

She was referring to the now-stalled denuclearization talks with North Korea also involving South Korea, the U.S., China, Japan and Russia.

As part of his efforts to win support from the other parties, U.S. Secretary of State John Kerry has held urgent phone talks with his South Korean, Chinese and Japanese counterparts, according to Nuland.

Beijing has joined in a chorus of condemnation against Pyongyang for its third known nuclear experiment but China reiterated a call for the international community to react in a calm and cool-headed manner.

Nuland, meanwhile, confirmed that North Korea had given an advance notice on its plan to carry out another underground test.

"We were advised.... through our usual channel," she said, adding that Pyongyang gave no specific timing prior to the event.

U.S. officials say a so-called New York channel remains open all the time, referring to North Korea's mission to the U.N.

In advance of a second nuclear test in 2009, North Korea also informally notified the U.S. of its plan, according to an informed source.