The top US military officer said Monday that any military strike against Iran would not be "decisive" in countering its nuclear program.
"No strike, however effective, will be in and of itself decisive,"Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, told a news conference, adding that he supported using diplomatic and economic pressure against Iran.
Also Monday, the United States said Monday that Iran's plan to build two new uranium enrichment plants is "further evidence" it rejects engagement with the international community.
State Department spokesman Philip Crowley added that the United States and other powers were reviewing Iranian targets for sanctions and Washington would offer specific proposals to the United Nations in the coming weeks.
President Barack Obama's administration has increasingly turned its attention to sanctions after its first-year bid to engage Iran in talks over its nuclear program and other issues yielded nothing concrete.
US officials say Iran's behavior shows it does not want to cooperate with the world community.
Iran has balked at an International Atomic Energy Agency (IAEA) offer to ship uranium abroad for enrichment, been forced to disclose a second uranium enrichment plant hidden in the mountains near Qom and defiantly announced plans for yet more enrichment plants.
'Change in behavior needed'
Iran -- which had mentioned plans for 10 new enrichment plants weeks ago -- said on Monday it is considering plans to build two of them concealed inside mountains to avert air strikes.
"This is further evidence that Iran refuses to engage cooperatively and constructively with the IAEA," Crowley told reporters, referring to the UN nuclear watchdog.
"Adding... more potential enrichment sites adds to the questions, rather than resolves the questions that the international community has," Crowley said.
Britain, China, France, Russia and the United States -- the permanent five members of the UN Security Council -- plus Germany continue to work closely "to identify potential targets for sanctions," he said.
"And we will, I think, be advancing specific proposals... to the UN in the coming weeks."
On Thursday, IAEA chief Yukiya Amano, in a blunt first report to the watchdog's board of governors, expressed concern that Iran might be seeking to develop a nuclear warhead.
"The IAEA report represents one of the clearest denunciations of what the Iranians have been working on," White House spokesman Robert Gibbs told reporters.
Gibbs said Obama and international leaders "have been clear that without a change in behavior, the Iranian government faces necessary consequences."
He suggested that China -- which has balked at punitive measures -- would eventually endorse a fourth round of UN sanctions against Iran.
"We've worked quite closely with the Chinese on the strongest sanctions that have ever gone through the Security Council in Resolution 1847 dealing with North Korea," he said.
"So we believe strongly, and I think the Chinese believe, that an arms race either in the Middle East or an international arms race is, in no way, in their interest," he said.