Iran has dismissed a report in the US media that US commandos are conducting missions on Iranian territory to identify targets for possible strikes.
Iranian officials and press are ridiculing claims that US commandos have operated inside Iran
Supreme National Security Council spokesman Ali Aghamohammadi said it would be naive to believe the report, which he called psychological warfare.
Earlier, Iranian Defence Minister Ali Shamkhani said Iran had the military strength to deter attacks.
US President George W Bush says he does not rule out force against Iran.
Washington has called on Tehran to reveal more about its nuclear plan, accusing it of developing nuclear weapons.
Iran insists it only wants to produce electricity.
The claims on US operations on Iranian territory were made by respected investigative journalist Seymour Hersh and published in the New Yorker magazine.
The Pentagon has described the report as based on rumour, innuendo and conspiracy theories, but has not categorically denied that US troops have been on the ground in Iran.
In the first official Iranian reaction to Mr Hersh's article, Mr Aghamohammadi told Iranian radio it was naive to imagine American spies could carry out secret reconnaissance missions inside Iran.
He said it was not easy for US commandos to enter the country.
His views were echoed by the Teheran Times which dismissed the story as a "ridiculous bluff".
It said the US and Israel knew that attacking Iran would be biting off more than they could chew, and would only choke them if they tried it.
The paper warned that Iran had acquired massive military might and was prepared to attack any intruder with a fearsome reign of fire and death.
Defence Minister Ali Shamkhani said Iran did not fear attack, in comments that were published on Tuesday but may not be a direct response to Hersh's article.
"We can claim that we have rapidly produced equipment that has resulted in the greatest deterrent," he said, without giving further details.