Iran is determined to have a nuclear weapon and could possess one within 10 years, according to the top US intelligence chief.
Director of National Intelligence John Negroponte told BBC Radio's Today programme Tehran could have a nuclear bomb ready between 2010 and 2015.
His comments come after key world powers agreed proposals in Vienna to persuade Tehran to halt its research.
The UK said the UN Security Council would act if Iran did not comply.
But on Friday Russian Foreign Minister Sergei Lavrov said the Vienna agreement meant that force was ruled out.
Iran resumed enrichment of uranium this year but remains a signatory to the nuclear Non-Proliferation Treaty and maintains that its activities are aimed only at energy production.
However in his interview with the BBC, Mr Negroponte said: "[Iran] seems to be determined to develop nuclear weapons.
"We don't have a clear-cut knowledge but the estimate we have made is some time between the beginning of the next decade and the middle of the next decade they might be in a position to have a nuclear weapon, which is a cause of great concern."
BBC diplomatic correspondent Jonathan Marcus says such predictions must cope with intelligence from Iran that is often patchy at best.
Iran is the world's top state sponsor of terrorism, says Mr Negroponte
It is as hard to determine who really is in control in Tehran as it is to decide what the government's nuclear intentions are, our correspondent says.
Mr Negroponte also accused Iran of being the world's top state sponsor of terrorism.
"Their behaviour has been a cause of concern not only in Lebanon and Israel and in the Palestinian territories but in Iraq," Mr Negroponte said.
He also acknowledged that US intelligence had made mistakes assessing the weapons capabilities of Iraq before the invasion, but said lessons had been learned and corrective measures taken.
He said: "I think we have improved the integrity of our analyses substantially."
Of the US-led war on terror, Mr Negroponte said it was "hard to estimate" when it would end. "I think we are talking about something that might go on for some period of time," he said.
In Vienna on Thursday, the Security Council permanent members plus Germany agreed a package of incentives and penalties to try to induce Iran to curb its nuclear programme.
A European Union diplomat said on Friday contacts were being made with Iran and EU foreign policy chief Javier Solana was on standby to deliver the proposals.
Sergei Lavrov says the military force option is ruled out
Also on Friday, Mr Lavrov said the Vienna accord meant there could be no military response, which the US has so far refused to rule out.
"I can say unambiguously that all the agreements... rule out in any circumstances the use of military force," Mr Lavrov told the RIA news agency.
He said if Iran rejected the proposals, the matter would return to the Security Council for a resolution but that this contained no call for sanctions.
Although no details have been made public of the Vienna accord, sources say it could include giving Iran a nuclear reactor and an assured supply of enriched uranium.
All Security Council action could also be suspended upon compliance.
US Secretary of State Condoleezza Rice said Iran's response would have to be "within weeks".
There has been no official reaction yet from Iran. But influential cleric Ahmad Khatami said at Friday prayers in Tehran: "The Iranian nation is ready to pay any price to protect its right [to nuclear technology]."
The US has so far refused direct talks with Iran over the issue although this week Ms Rice said Washington would come to the table when Iran fully and verifiably suspended its enrichment and reprocessing activities.
Iranian Foreign Minister Manouchehr Mottaki welcomed the idea of talks with the US but ruled out any compromise on enrichment.