Talks on Wednesday with the European Union about Iran's nuclear programme have only a 50-50 chance of success, a leading Iranian negotiator has warned.
The next round of talks could be the last chance to find a solution
Speaking after preparatory talks in Brussels, Hossein Mousavian said there were no guarantees of avoiding a breakdown in negotiations.
"These talks were more difficult and complicated than ever," he said.
The EU has threatened to back a US call for United Nations sanctions if Iran resumes the enrichment of uranium.
The US accuses Tehran of developing nuclear weapons. Iran insists its nuclear programme is for civilian use only.
So far, the EU approach has contrasted with the US hard line by offering economic and political incentives to keep enrichment activities frozen, in line with a deal struck in November.
The meeting in Paris on Wednesday is set to include the foreign ministers of the UK, France and Germany, along with the EU foreign policy head Javier Solana.
Mr Mousavian, a member of Iran's Supreme National Security Council, is one of the leading negotiators on the Iranian side, which is led by Hassan Rowhani.
The talks were called following threats by Iran to resume enriching uranium. It says it is entitled to do so, but is ready to offer guarantees that its nuclear programme will be exclusively for civilian purposes.
Despite attempts at rapprochement, EU leaders have warned Tehran that a breakdown in talks could lead to a referral to the UN Security Council.
That, foreign ministers said in a letter to Mr Rowhani, "would bring the negotiating process to an end", according to AFP news agency.
"The consequences beyond could only be negative for Iran," the letter said.
Iran went further in ratcheting up the temperature ahead of Wednesday's meeting.
"Tomorrow's session could bring an end to the extensive talks... if (the Europeans) don't have a clear proposal," said foreign ministry spokesman Hamid Reza Asefi.
"Or they can be a turning point in relations and co-operation between Iran and Europe. There's no possibility between these two options."