The struggle to end a 12-year dispute over Iran’s nuclear ambitions is into the last 48 hours as talks continue in Vienna.
US Secretary of State John Kerry said “big gaps” remained between the positions of Tehran and six world powers.
A European source said the chance of a final deal by Monday’s self-imposed deadline was “very small”.
Western officials say Iran still refuses to curb uranium enrichment in return for having sanctions lifted.
The US Senate with a new Republican majority is likely to harden its anti-Iranian stance, warning against concessions.
“The Iranians have a long record of cheating, concealing their nuclear capabilities, and they have had to be uncovered from time to time,” Republican Senator John McCain told euronews.
“If they were serious they would not want to enrich. They continue to develop the warhead and the missiles to deliver nuclear weapons. We are on the verge, if the administration has its way, of a very bad deal,” the former presidential candidate added.
Iran also faces domestic pressure not to make concessions.
The supreme leader Ayatollah Ali Khamenei has the final say on a deal, and has repeatedly warned that he’s pessimistic over the chances of reaching one.
But he has backed Iran’s negotiating team, keeping in check hardliners.
They include Revolutionary Guards, clerics and conservative politicians, and oppose any compromise on Iran’s right to enrich uranium – a process that can have both civilian and military uses.
Tehran rejects Western accusations that it’s trying to develop a nuclear bomb.
But some reformist publications argue it would be in Iran’s interest for a deal to be signed.
One influential paper, Javan – believed to be close to Iran’s Revolutionary Guards – has written that the country’s ideal outcome of a complete lifting of sanctions will not be reached.