European Union officials have called for an emergency meeting of the UN nuclear watchdog to discuss the stand-off over Iran's programme.
The nuclear plant at Isfahan is at the centre of the current row
The International Atomic Energy Agency (IAEA) board of governors is due to take place next week but the date has not been set, a spokesman said.
Britain, France, Germany and the EU have warned Iran against re-starting the enrichment of uranium.
They said this would end two years of talks on Iran's nuclear activities.
Earlier on Thursday, Iran's top nuclear negotiator accused EU foreign ministers of making factual errors and unacceptable threats over Iran's nuclear programme.
Hassan Rowhani said that since Iran's suspension of some of its nuclear activities was voluntary, there would be nothing illegal about resuming them.
Behind the European letter is the threat to refer the whole issue to the United Nations Security Council, which might lead to sanctions against Iran.
The United States, which accuses Iran of wanting to develop nuclear weapons, has been seeking this course of action for some time.
Mr Rowhani said uranium enrichment activities would remain suspended for the time being.
He accused the Europeans of failing to respond to Iran's proposal to give objective guarantees which would allow it to continue developing nuclear technology.
"The three European ministers have said that if we restart [nuclear activities] this would mean the end of negotiations. This is a threat, this is unacceptable," said Mr Rowhani.
"There is no judicial or political logic to send the issue to the UN Security Council, this would mean that the Europeans have given in to US pressure and they must assume the consequences," he said.
"Once the Isfahan plant restarts, we want to continue the negotiations with the Europeans."
In their letter, the Europeans had questioned why Iran was in such a rush to produce its own nuclear fuel, when it currently had no operational nuclear power plants.