US Defence Secretary Robert Gates has warned that Iran still poses a serious threat to the Middle East and the US.
The US report confirmed Iran had a covert programme, Mr Gates said
Mr Gates told a Bahrain conference Iran may have restarted its nuclear weapons programme, despite a US intelligence report saying it had stopped.
He urged Gulf states to put pressure on Iran to "come clean" about past nuclear ambitions and stop uranium enrichment.
US efforts to get Iran to suspend enrichment were 100% diplomatic and economic, he said.
But all options were on the table, he added.
The Iranian foreign minister, Manouchehr Mottaki, had been due to attend the conference, but cancelled at the last moment.
Iran has always insisted the aims of its nuclear programme were peaceful.
Fresh from his recent visit to US troops in Iraq, Afghanistan and the Horn of Africa, Mr Gates told an audience of Gulf Arab defence and security chiefs that Iran remained a serious threat to the region.
"Everywhere you turn, it is the policy of Iran to foment instability and chaos, no matter the strategic value or cost in the blood of innocents - Christians, Jews and Muslims alike," he said, at a security conference in the Bahraini capital, Manama.
"There can be little doubt that their destabilising foreign policies are a threat to the interests of the United States, to the interests of every country in the Middle East, and to the interests of all countries within the range of the ballistic missiles Iran is developing."
Mr Gates acknowledged that last week's US National Intelligence Estimate, which concluded Iran had halted its nuclear weapons programme four years ago, had caused problems for President Bush and confused America's allies.
The report had come at an awkward time, he said, but both the timing and the content were determined by the US director of national intelligence, not the government.
It showed, said Mr Gates, that Iran did have a covert nuclear weapons programme hidden for years and that it had the mechanism to restart it at any time, if indeed, he added, Iran had not done so already.
Iran was also funding militias in Iraq and supporting Islamist organisations like Hamas and Hezbollah, he said.