European countries are calling on Iran to cease uranium enrichment permanently because the process could be used to produce material for nuclear weapons.
Iran agreed to halt enriching uranium last November
Britain, France and Germany are negotiating with Iran at the UN's nuclear watchdog the International Atomic Energy Agency.
The EU countries are reported to be offering political and economic incentives as part of any deal.
Tehran agreed to a temporary freeze on enrichment last November.
But it has repeatedly said it will never give up its right to full mastery of the nuclear fuel cycle.
A confidential EU document said the European trio would not settle for anything less than a complete cessation.
"Iran has to recognise that the fuel cycle programme is the core of the problem," the document said, Reuters news agency reported.
But a senior member of Iran's negotiating team, Hossein Mousavian, told the official Irna news agency that the talks were not about terminating the programme.
"The core of the discussions is not the cessation of enrichment but rather giving objective guarantees about Iran's fuel cycle not being diverted to make atomic bombs," he said.
Washington says Iran is secretly developing nuclear weapons - Iran says its nuclear programme is for purely peaceful purposes.
Correspondents said the tough-sounding tone of the EU document will please the United States, which is warning that it might resort to military force to tackle the issue.