The US has rejected suggestions that it might be preparing to use nuclear weapons against targets in Iran.
Western powers fear Iran is developing a nuclear bomb
A report in The New Yorker magazine said the US was increasing planning for a possible air attack on Iranian nuclear facilities.
It said one option being considered was a tactical nuclear strike against underground nuclear sites.
Dan Bartlett, a senior adviser to President George W Bush, said the report was "ill-informed".
Those who drew definitive conclusions based on normal defence and intelligence planning "are not knowledgeable of the administration's thinking on Iran", he said.
The US has previously refused to rule out military action, but Mr Bartlett said again that the US was committed to a diplomatic solution on the issue of Iran's nuclear development.
UK Foreign Secretary Jack Straw said talk of a US nuclear strike was "completely nuts".
Iran has branded the reports as a "psychological war launched by Americans because they feel angry and desperate regarding Iran's nuclear dossier".
"We will stand by our right to nuclear technology... Iran is not afraid of threatening language," Foreign Ministry spokesman Hamid Reza Asefi said on Sunday.
Western powers fear Iran is developing a nuclear bomb. Iran says its nuclear programme is for civilian use.
The US magazine article, by journalist Seymour Hersh, makes three main claims:
- US clandestine activities inside Iran have increased
- Planning for a possible air attack has intensified
- The option of using of tactical nuclear weapons to ensure the destruction of well-protected Iranian nuclear facilities is still on the table.
Mr Hersh also quoted a former senior intelligence official as saying that President Bush and others in the White House were referring to Iranian President Mahmoud Ahmadinejad as a potential Adolf Hitler threatening another world war.
Speaking to the BBC, Mr Hersh said many US allies felt Iran was two to 10 years away from developing a nuclear bomb and that the real aim was regime change.
"No matter what Iran would do, I think in the short run some people are afraid the president will want to go - just as he wanted to go on Iraq," he said.
He said he believed the president felt military action against Iran was something only he could do. "It's messianic, I quote somebody as saying," he said.
Planning for military action had moved beyond the contingency stage and into direct operational planning, he added.
Referring to Mr Straw's comments, he said there were questions about how much information the US government was sharing with its European allies.
Retired General Anthony Zinni, the former head of US Central Command, said on Sunday any plan to use military force against Iran was risky.
The US says it will not rule out a military option in Iran
"The Iranians will retaliate, and they have many possibilities in an area where there are many vulnerabilities, from our troop positions to the oil and gas in the region that can be interrupted, to attacks on Israel, to the conduct of terrorism," he told the Associated Press news agency.
Talk of military strikes against Iran have been prompted by Iran's refusal to halt nuclear work.
Last month, the UN Security Council gave Iran 30 days to halt its nuclear research, or run the risk of action such as possible sanctions.