Iran has warned that there is only a 50% chance of success in crunch nuclear talks with European Union negotiators this week.
The talks are aimed at avoiding an escalation of Tehran's standoff with the West.
Speaking after official-level talks to prepare for formal negotiations in Geneva tomorrow, a top Iranian official was blunt about the difficulties.
Hossein Moussavian said: 'We are at the most difficult part of the negotiation. There is a 50-50 chance of success... of reaching a compromise between the two sides. We have had some steps forwards, but we have a lot to go.'
The 25-nation EU has warned that it could refer Iran to the UN Security Council - and into Washington's diplomatic line of fire - if the talks fail.
The foreign ministers of Britain, France and Germany and EU foreign policy chief Javier Solana will meet top Iranian negotiator Hassan Rohani in Geneva tomorrow.
Iran has said it plans to resume operations at its uranium enrichment plant, in breach of an accord to suspend them last November.
The EU has insisted that this would mean the collapse of compromise negotiations.
