"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."
Exclusive: Obama Calls on China to Take Stand Against North Korean Attack
There were no U.S. forces involved in the annual South Korean training exercises, according to the Pentagon. In years past, U.S. Marines have participated in the exercise, but a scheduling conflict prevented their participation this year. Planning is still underway for a joint U.S.-South Korean exercise in the Yellow Sea, but the timing hasn't been announced.
Defense Secretary Robert Gates called his South Korean counterpart today and reiterated the U.S. stance against "this act of aggression."
"In a phone call this morning, Secretary Gates told Minister Kim the United States strongly condemns the attack by North Korea, views it as a violation of the armistice agreement and assured him that we are committed to South Korea's defense," Pentagon Press Secretary Geoff Morrell told reporters. "He expressed sympathy for the loss of life and appreciation for the restraint shown to date by the South Korean government."
The exchange of fire came days after North Korea revealed its upgraded and strengthened uranium enrichment plant to western scientists.
Siegfried Hecker, the Stanford professor and former head of the Los Alamos lab who was invited to North Korea last week to witness their new uranium program, said today the country's nuclear capability is much farther along than previously estimated.
He added that the Obama administration should undertake a thorough review of North Korea, since the last one was in 2000.
ABC News' Joohee Cho, Jake Tapper, Kirit Radia, Luis Martinez and The Associated Press contributed to this report.