A military strike will not completely stop Iran's nuclear program, Chairman of the Joint Chiefs of Staff Admiral Mike Mullen said in a Pentagon briefing on Monday.
"No strike, however effective, will be in and of itself decisive," Admiral Mike Mullen said, adding that he supported using diplomatic and economic pressure against Iran.
Iran's uranium enrichment, in defiance of several rounds of Security Council sanctions, has spurred world powers to consider tougher diplomatic measures, against the backdrop of threatened military action by Israel as a last resort.
When asked on what steps the U.S. intended to take to thwart Iran's nuclear ambitions, State Department spokesman Philip Crowley told reporters Monday that Washington was "continuing to look at other steps that we can take both multilaterally and prospectively on a national basis."
Crowley added that the U.S. was "looking at the full range of possibilities."
"We want to see effective sanctions that have the impact that we want to put pressure on Iran. And as the Secretary has said, we will be paying specific attention to the Revolutionary Guard Corps that is playing a more - a growing role in Iranian society and in the Iranian economy," Crowley said.
Earlier Monday, a nuclear energy official said that Iran has already earmarked potential sites for new nuclear enrichment plants and construction of two of them could begin this year.
"We have earmarked close to 20 sites and have passed the report on those to the president, however, these sites are only potential," Ali Akbar Salehi, head of the Atomic Energy Organization, was quoted as saying on news agency ISNA.
"We should begin the construction of two enrichment sites next year ... In the two new sites, we plan to use new centrifuges. "The next Iranian year begins on March 21.
Washington fears Iran's nuclear energy program will allow Tehran to develop nuclear weapons. Iran denies any such intention and says it only wants to generate electricity.
The head of U.S. Central Command, General David Petraeus, told NBC's "Meet the Press" on Sunday that the United States is placing its efforts to thwart the Iranian nuclear program on a "pressure track."
Petraeus said that he thought "that no one at the end of this time can say that the United States and the rest of the world have not given Iran every opportunity to resolve the issues diplomatically."
"That puts us in a solid foundation now to go on what is termed the pressure track," the U.S. general said, adding that "that's the course on which we are embarked now."
On Monday, Prime Minister Benjamin Netanyahu called foran immediate embargo on Iran's energy sector, saying the United Nations Security Council should be sidestepped if it cannot agree on the move.
Netanyahu told foreign Jewish leaders that if the world "is serious about stopping Iran, then what it needs to do is not watered-down sanctions, moderate sanctions ... but effective, biting sanctions that curtail the import and export of oil into Iran."