Although already planned for months, the White House described the pending arrival of the USS George Washington aircraft carrier strike group to South Korean waters as a timely demonstration of "the strength of the ROK (Republic of Korea)-U.S. Alliance and our commitment to regional stability through deterrence."
In a statement released Tuesday night, the White House said the ship, along with five other U.S. war ships, would arrive off the Korean peninsula on Nov. 28 and participate in long-planned military exercises with the South until Dec. 1. Such U.S.-South Korean military drills are often labeled by the North as provocation.
A Pentagon official later said the exercises should be seen as a deterrent to North Korea, even though they were planned before the attack, reports CBS News White House correspondent Mark Knoller.
The visible show of support for South Korea followed strong condemnations from White House of the North's attack, which the U.S. branded as provocative and outrageous.
S. Korea Decries "Inhumane Atrocities" in Clash
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"South Korea is our ally. It has been since the Korean war," Mr. Obama said in his first comments about the North Korean shelling of a South Korean island. "And we strongly affirm our commitment to defend South Korea as part of that alliance."
The White House issued a statement later Tuesday that read in part, "This evening, President Obama called President Lee to tell him that the United States stands shoulder to shoulder with our close friend and ally, the Republic of Korea. The President strongly condemned the attack by North Korea on the South Korean island of Yeonpyeoung, which took the lives of at least two Koreans and injured many more."
Washington has relatively few options when dealing with Pyongyang. Military action is particularly unappealing, since the unpredictable North possesses crude nuclear weapons as well as a huge standing army. North Korea exists largely outside the system of international financial and diplomatic institutions that the U.S. has used as leverage in dealing with other hostile countries, including Iran.
Working to head off any escalation, the U.S. did not reposition any of its 29,000 troops in the South or make other unplanned military moves after North Korea fired salvos of shells into the island, setting off an artillery duel between the two sides.
The president, speaking to ABC News, would not speculate when asked about military options. He met earlier with his top national security advisers to discuss next steps.
The administration official said the U.S. did not interpret North Korea's aggression as a desire to go to war, but as yet another effort to extract concessions from the international community.
"The North Koreas have undertaken over time a number of provocations that have manifested themselves in different ways," Air Force Chief of Staff Gen. Norton Schwartz said.
South Korean officials said Tuesday's clash came after Pyongyang warned the South to halt military drills near the small South Korean island of Yeonpyeong.
When Seoul refused and began firing artillery into the water near the disputed sea border, the North bombarded Yeonpyeong, which houses South Korean military installations and a small civilian population. The shelling killed at least four - two South Korean marines and two more civilian, whose bodies were later.
The artillery exchange was only the latest serious incident between the two nations. In March, a South Korean naval ship, the Cheonan, exploded and sank in the Yellow Sea, killing 46 sailors. South Korea accused the North of torpedoing the vessel; the North denied the allegation.
In August, the South Korean military reported that the North had fired 110 artillery rounds into the Yellow Sea near the disputed sea border but said the shells fell harmlessly into North Korean waters.
Recent joint U.S.-Korean naval exercises and strenuous denunciations of the North may only have provoked the regime in Pyongyang. Some experts say the secretive regime may be trying to promote who, like his father, is capable of standing up to the U.S.
"I think it may be all wrapped in this succession planning, in the way the North is looking at it," said Robert RisCassi, a retired Army general who commanded U.S. forces in Korea from 1990-93.
The U.S.-South Korea exercises also angered China. Beijing is regarded as the key to any long-term diplomatic bargain to end North Korea's nuclear program and reduce tensions on the peninsula.
But U.S. officials say the North's motives and internal politics are opaque and sometimes appear inconsistent.
"I don't know the answer to any question about North Korea that begins with the word 'why,'" Defense Secretary Robert Gates told reporters Monday.
North Korea has resisted pressure from its major ally, China, which appears to be nervous about the signs of instability in its neighbor.
An administration official said Tuesday evening that U.S. officials in Washington and in Beijing were appealing strongly to China to condemn the attack by arguing that it was an act that threatened the stability of the entire region, not just the Korean peninsula. The official spoke on the condition of anonymity because of the sensitivity of the matter.
Gates phoned South Korea's defense minister to express sympathy for the deaths of two of the South's marines in the artillery shelling of a small South Korean island and to express appreciation "for the restraint shown to date" by the South's government, a Pentagon spokesman said.
Mr. Obama called North Korea's action "just one more provocative incident" and said he would consult with Lee on an appropriate response.
In his phone call to South Korea's defense minister, Gates said the U.S. viewed recent attacks as a violation of the armistice agreement that ended the Korea War in 1953, and he reiterated the U.S. commitment to South Korea's defense, said Pentagon press secretary Geoff Morrell.
Mr. Obama was awakened at 4 a.m. Tuesday with the news. He went ahead with an Indiana trip focused on the economy before returning to the White House after dark.
State Department spokesman Mark Toner said the U.S. would take a "deliberate approach" in response to what he also called provocative North Korean behavior. At the same time, other administration officials, speaking on condition of anonymity to describe the emerging strategy, said the White House was determined to end a diplomatic cycle that officials said rewards North Korean brinksmanship.
In the past, the U.S. and other nations have sweetened offers to North Korea as it has developed new missiles and prototype nuclear weapons. North Korea is now demanding new one-on-one talks with the United States, which rejects that model in favor of group diplomacy that includes North Korea's protector, China.
"We're not going to respond willy-nilly," Toner said. "We believe that it's important that we keep a unified and measured approach going forward."
Both Republicans and Democrats on Capitol Hill accused North Korea of starting the skirmish.