Iran will strike against US interests worldwide if it is attacked, the country's supreme leader Ayatollah Ali Khamenei has warned.
Ayatollah Khamenei says Iran has a right to nuclear power
"The enemies know well that any aggression will lead to a reaction from all sides," he said.
Washington accuses Tehran of secretly trying to develop a nuclear weapon, and has not ruled out using military force.
The Iranians insist their nuclear programme is purely civilian and aimed at meeting their energy needs.
The BBC's Frances Harrison in Tehran says Ayatollah Khamenei was defiant about the prospect of a possible American military strike.
The supreme leader said he hoped nobody would risk attacking Iran because the nation would stand up for itself and only become stronger militarily and economically.
Iran also denounced remarks by UK Prime Minister Tony Blair that Tehran was determined to stir up maximum trouble in the Middle East.
Iranian foreign ministry spokesman Mohammed Ali Hosseini said Mr Blair's comments were "insolent" and "undiplomatic".
Mr Hosseini said Britain had played a key role in sabotaging talks on the nuclear issue in the past and had followed the US and Israel in imposing destructive wars on the Middle East.
Another key Iranian figure, ex-President Hashemi Rafsanjani, has also warned against a strike, saying it would carry a heavy cost for those who tried it.
The warnings came as Iran's navy and air force conducted war games.
Diplomats were shown the Isfahan nuclear plant at the weekend
Iran said it had successfully test-fired a land-to-sea missile with a range of 350km (220 miles).
Tehran said it had also tested a new Russian-made air defence system.
Officials have refused to confirm whether the system has been deployed around nuclear sites.
At the weekend ambassadors from non-aligned countries were allowed to visit an Iranian nuclear facility, on what was billed as a transparency visit.
The UN's chief nuclear inspector is to report on Tehran's compliance with the UN Security Council's demands later this month.
In December the UN imposed limited sanctions on Iran for refusing to suspend uranium enrichment.