French Foreign Minister Michel Barnier has urged Iran not to resume suspended nuclear operations, warning that to do so would lead to "consequences."
Iran said the suspension of nuclear activities was only temporary
Officials at the UN atomic agency say Iran is backing away from a plan to resume uranium enrichment, which could be used to develop nuclear arms.
A diplomat quoted by the AP news agency said Tehran had been dissuaded from the plan by the European warning.
The consequences are assumed to be action by the UN Security Council.
"We continue to hope that Iran will not take this step, the consequences of which it is well aware," Mr Barnier said.
He said such a move would "be counter to the Paris agreement and resolutions adopted by the IAEA. We want to get this position across to the authorities in Tehran".
Negotiations have been taking place between Iran and three European countries, Britain, France and Germany, to try to agree a way of ensuring that Iran's nuclear programme is entirely peaceful.
An Iranian news agency says the country's top nuclear negotiator, Hassan Rohani, has warned that Iran will no longer respect the Nuclear Non-Proliferation Treaty if it is prevented from using nuclear technology for peaceful purposes.
"If Iran cannot exercise its rights with in the framework of the NPT, it will no longer have any respect for this treaty," Mr Rohani said.
Mr Rohani told state television that Iran had received messages from the Europeans, but said the decision to resume work at its Isfahan nuclear plant was definite.
"Iran will definitely resume a part of its enrichment activities in the near future... but we are still discussing its conditions and time of restarting the activities."
Iran has been saying for the last two weeks that it intends to resume conversion of raw uranium into gas at its Isfahan plant, a move that would give it the capability to produce enriched uranium for a nuclear bomb.
A senior Iranian nuclear negotiator has been sent to the UN agency, the IAEA, in Vienna with a letter.
Some reports say the letter, which has not been delivered, contains formal notification that Iran is about to break United Nations seals on the Isfahan plant.
The breaking of the seals would, correspondents say, signal the collapse of the negotiating process on Iran's nuclear programmes.
The basis of the talks was a promise by Iran to suspend all its nuclear activities, including uranium enrichment.
If that fails there is the prospect of referral to the United Nations Security Council.
Correspondents say that Iran may be engineering a controlled crisis to obtain more concessions in the talks, but Western officials warn that this is misreading the mood in Europe, which is not willing to tolerate Iran violating its agreements.
Iran, however, says it agreed to suspend its nuclear activities only on a short-term basis and on the assumption that negotiations were progressing well.