The United States is calling North Korea's sinking of a South Korean warship an act of aggression which will further isolate Pyongyang. Top U.S. officials are not calling the attack an act of war or state-sponsored terrorism.
Officials in Washington are accepting an international report which concludes that it was a North Korean torpedo which sank a South Korean warship in March.
Forty-six South Korean sailors were killed in the country's worst military disaster since an armistice halted the Korean War in 1953.
North Korea denies sinking the ship, and says retaliation would mean all-out war.
White House spokesman Robert Gibbs says the U.S. is consulting with South Korea, its Asian neighbors and the United Nations on how to respond.
North Korea is still under sanctions from last year's nuclear and missile tests. Gibbs says the sinking of the South Korean ship will further isolate North Korea from the international community.
"That is what resulted in a very strong set of sanctions last year, and this act of aggression, this clear violation of the armistice agreement, further sets them back and further isolates them," said Robert Gibbs.
U.S. State Department spokesman P.J. Crowley says there will be consequences for the attack, which he says has raised tensions on the Korean peninsula and beyond.
"This was a provocative, unprovoked and unwarranted action by North Korea," said P.J. Crowley. "We, of course, have concern at all times that these kinds of irrational actions by North Korea potentially can provoke a broader crisis."
Secretary of State Hillary Clinton left Thursday for a visit to Asia. Crowley says North Korea will be high on the agenda when she meets with officials in Japan, China and South Korea.
The Chairman of the U.S. military Joint Chiefs of Staff, Admiral Mike Mullen, says American forces in and around South Korea have not been put on a higher level of alert since the attack. The top U.S. military officer says, however, officials remain vigilant.
"We are all focused on that region, the stability in that region that needs to be sustained, and at the same time, very focused on supporting our strong ally in the Republic of Korea," said Admiral Mullen.
U.S. Defense Secretary Robert Gates says South Korea will lead the decision on how to deal with the situation.
"This was an attack on a South Korean ship, and the South Koreans need to be in the lead, in terms of proposing ways forward," said Robert Gates.
The United States has about 28,000 troops in South Korea, but officials in Washington are being cautious.
At the State Department, spokesman P.J. Crowley says the U.S. has no interest in seeing the Korean peninsula explode.