Tehran says media reports that the US may be preparing to use nuclear weapons against targets in Iran are part of a psychological warfare campaign.
President Ahmadinejad again defended his nation's nuclear policy
A report in The New Yorker magazine said the US was increasing its planning for a possible air attack on Iranian nuclear facilities.
A senior Iranian presidential aide said the report was a joke and that any such attack was highly unlikely.
US President George W Bush dismissed the report as "wild speculation".
Iranian President Mahmoud Ahmadinejad on Monday maintained his country's strong stance on its nuclear programme, saying it would not back down "one iota" on demands that it freeze its uranium enrichment programme.
President Ahmadinejad also promised "very good nuclear news in the coming days", without giving more details.
That news could coincide with the visit on Wednesday of Mohamed ElBaradei, the head of the UN's nuclear watchdog, the International Atomic Energy Agency.
In the New Yorker report, veteran journalist Seymour Hersh said the US had stepped up plans for a possible air attack and had increased clandestine activities inside Iran.
Iran's top nuclear negotiator, Ali Larijani, said the report was part of a White House-orchestrated psychological warfare campaign against Iran's nuclear programme.
He said if Washington was serious about such plans they would keep them secret.
Iranian army chief of staff General Abdolrahim Mousavi told the Isna news agency Iran would strike back if attacked.
"We will certainly retaliate against any action by the enemy, and we are vigilant over any military aggression," he said.
Senior presidential aide Hamid Reza Taraqi said a tactical nuclear strike was highly unlikely and he rejected claims US combat troops might already be inside Iran making contact with ethnic minority groups.
Mr Taraqi said it was clear the Americans had no idea what was going on inside Iran.
But BBC Iranian affairs analyst, Sadeq Saba, says despite the official Iranian reactions, such reports will cause some concern among ordinary Iranians.
The Iranian people have witnessed two major wars in neighbouring Afghanistan and Iraq in recent years and they are increasingly fearful they may face a similar fate if Iran's nuclear crisis is not resolved through diplomatic means, our correspondent says.
The US, Britain and the EU all dismissed the New Yorker report.
Speaking in Washington, Mr Bush insisted the US was committed to finding a diplomatic solution to the Iran issue.
"I know here in Washington prevention means force. It doesn't mean force necessarily. In this case it means diplomacy.
"What you are reading is just wild speculation which happens quite frequently here in the nation's capital."
Western powers fear Iran is developing a nuclear bomb
UK Foreign Secretary Jack Straw said talk of a US nuclear strike was "completely nuts".
EU foreign policy chief Javier Solana said: "I have read the article... I think it has nothing to do with reality."
On Monday Iranian President Ahmadinejad continued to defend Iran's nuclear policy.
Last month, the UN Security Council gave Iran 30 days to halt its nuclear research, or run the risk of action such as sanctions.
But Mr Ahmadinejad said in a televised speech Iran would not back down.
"Our enemies know they are unable to even slightly hurt our nation... They should know they cannot deprive our nation of its rights by political pressure."
Mr Hersh had quoted a former senior intelligence official as saying that President Bush and others in the White House were referring to Mr Ahmadinejad as a potential Adolf Hitler.
Western powers fear Iran is developing a nuclear bomb. Iran says its nuclear programme is for civilian use.