Iran has strongly criticised the US at the United Nations, accusing Washington of threatening to launch a military strike against its nuclear facilities.
Many Iranians say it is their right to have nuclear power
In a letter delivered to Secretary General Kofi Annan, Iran said the US was "openly" planning to attack Iran in breach of international law.
Officials from the five permanent members of the UN Security Council will discuss Iran in Paris on Tuesday.
Iran has resisted calls by the UN for it to suspend uranium enrichment.
The rhetoric between the US and Iran has become more heated following the UN nuclear watchdog's report on Friday which found Tehran had not complied with a UN demand to end uranium enrichment, says the BBC's Laura Trevelyan at the UN.
Tehran insists it has the right to a peaceful nuclear energy programme, although the US accuses it of trying to develop atomic weapons.
In the letter to Mr Annan, Iran's ambassador to the UN criticised US President George W Bush, who has refused to rule out even a nuclear strike on Iranian targets.
US rhetoric amounted to "illegitimate and open threats to use force against the Islamic Republic of Iran," the letter said.
IRAN CRISIS: NEXT STEPS
2 May: Negotiators from US, Russia, China, UK, France and Germany meet in Paris
3 May: Possible Security Council meeting to discuss IAEA report
9 May: Foreign ministers from US, Russia, China, UK, France and Germany meet at UN
"These are in obvious contravention of international rules and the principles of the United Nations."
The letter referred specifically to a report in the New Yorker by journalist Seymour Hersh, which suggested US military planners were considering a "bunker-busting" nuclear strike.
Mr Bush has refused to rule out military action, but has repeatedly insisted that the dispute should be resolved diplomatically.
The Iranian letter was delivered at the start of a busy week of diplomacy for the five permanent members of the UN Security Council.
Mr Bush discussed the Iranian issue during a phone call with Russian President Vladimir Putin on Monday, officials said.
Along with China, Russia has been reluctant to pressure Tehran into concessions, and is opposed to the suggestion of sanctions or military action.
Foreign policy officials from the US, UK, Russia, China and France are to meet in Paris on Tuesday to discuss how to approach the Iranian issue.
The council itself could meet a day later to discuss the report issued last week by the UN's nuclear watchdog, the IAEA, which judged Iran to be in breach of UN requests that it halt enrichment.
A new Security Council resolution could be circulated that would make legally binding all previous calls for Iran to stop enriching uranium.
Foreign ministers from the five permanent members will then meet at UN in New York on 9 May.