Exclusive: President Obama Says North Korea Poses a 'Serious and Ongoing Threat'

WASHINGTON, Nov. 23, 2010— -- President Obama today strongly condemned North Korea's attack on South Korea and, in an exclusive interview with ABC News' Barbara Walters, urged China to take a stand against aggression.

"This is a -- just one more provocative incident in a series that we've seen over the last several months, and I'm going to be talking to the president of Korea -- South Korea this evening and we'll be consulting closely with them in terms of the appropriate response," the president said. "We've strongly condemned the attack...We are rallying the international community once again to put pressure on North Korea."

Obama wouldn't speculate on military actions the U.S. may take, but reiterated that South Korea is "one of our most important allies" and "a cornerstone of U.S. security in the Pacific region."

"We want to make sure all the parties in the region recognize that this is a serious and ongoing threat that has to be dealt with," the president added.

He specifically called on China to stand firm and "make clear to North Korean that there are a set of international rules that they need to abide by."

A White House official today told ABC's Jake Tapper that the United States is discussing a number of measures with its allies, including action at the United Nations Security Council and further sanctions, and more joint U.S.-South Korea military exercises, to demonstrate solidarity and support.

Watch more of Barbara Walters' interview with the Obamas tomorrow on "Good Morning America" and Friday, Nov. 26 at 10 p.m. ET on "A Barbara Walters Special: A Thanksgiving Visit with President and Mrs. Obama"

Tensions escalated on the Korean peninsula this morning after North Korea fired artillery shells at South Korea's Yeonpyeong Island. The South Koreans responded with their own artillery fire.

Authorities said two South Korean soldiers and at least three civliians were killed. They said 16 others were injured.

South Korean naval forces were conducting a routine drill in the waters near the island earlier in the morning, which authorities say may have triggered a reaction from North Korea.

The western sea border has been at the center of dispute where the two Koreas fought bloody skirmishes in 1999, 2002, and most recently November 2009. But this is the first time since the end of the Korean war in 1953 that North Korea has fired on South Korea's civilian territory.

U.N. Secretary General Ban Ki Moon called the attack one of "gravest incidents since the end of the Korean War" and said he "is deeply concerned by the escalation of tension on the Korean peninsula."