International Atomic Energy Agency chief Mohamed ElBaradei said he will seek to urge top Iranian officials to put an end to sensitive work, less than two days after Tehran announced it had completed nuclear enrichment work.
"The situation is completely changed. We are a nuclear state," said hardline President Mahmoud Ahmadinejad.
"Our answer to those who are angry about Iran obtaining the full nuclear cycle is one phrase, we say: Be angry and die of this anger," said Mr Ahmadinejad said, reported by the official IRNA news agency.
"We will not hold talks with anyone about the Iranian nation's right (to enrichment) and no one has the right to step back, even one iota."
His triumphant declaration on Tuesday that Iran had enriched uranium to a level used in power stations ands that it wants to expand production drew swift condemnation from world powers, including Russia and China.
The Islamic republic insists its program is peaceful, but the enrichment process can be extended to make the fissile core of a nuclear warhead, and the West fears that Iran's program will be used to make weapons.
The UN Security Council has set April 28 as a deadline for Tehran to halt enrichment.
"We will not negotiate on our rights with anyone," Mr Ahmadinejad was quoted as saying by the official news agency IRNA, adding that nobody in the country "has the right to step back one iota from the path we are following."
Mr ElBaradei has arrived in Tehran for talks with the regime's top nuclear negotiator.
"We hope to convince Iran to take confidence-building measures including suspension of uranium enrichment activities until outstanding issues are clarified," Mr ElBaradei told journalists at the airport.
"I would like to see Iran has come to terms with the request of the international community," he said, adding that he still remains "hopeful the time is right for political solutions, through negotiations."
Mr ElBaradei must give a report at the end of April on Iran's nuclear activities to the UN Security Council and the 35 states of the IAEA's governing council.
Representatives of the five permanent members of the Council plus Germany are to meet in Moscow next Tuesday to discuss the crisis, with the long-running stand-off set to enter a period of far more robust diplomacy.
"Time for sanctions"
US Secretary of State Condoleezza Rice called on the 15-member Security Council to take "strong steps", and the White House said sanctions are now an option.
"It's time for the Security Council to act on the diplomatic front," said White House spokesman Scott McClellan.
"There are a number of options that are available to us through the diplomatic process," he said, adding that officials are still pursuing a diplomatic solution.
Officials from permanent Security Council members Britain, France and Russia and Germany, all said Iran had taken a "step in the wrong direction".
But oil-rich Iran has vowed it can weather any sanctions, and has vowed to go the other way, instead seeking to accelerate the process and reach an industrial-scale fuel production capacity.