Major powers and Iran were pushing each other for concessions on Friday ahead of an end-March deadline for a preliminary nuclear deal, with Tehran demanding an immediate end to sanctions and freedom to continue sensitive atomic research, officials said.
Tehran and six major powers - the United States, Britain, France, Germany, Russia and China - are meeting in Lausanne, Switzerland, to hammer out a political framework accord by the end of this month that would lay the foundations for a full deal by June 30.
Under a final settlement, Tehran would halt sensitive nuclear work for at least a decade and in exchange, international financial and oil sanctions on Iran would be lifted. This would aim to end the country's 12-year nuclear standoff with the West and reduce the risk of war in the Middle East.
While all sides agree they have been inching closer to a deal, there are major disagreements that have prevented a resolution.
Tehran insists on the freedom to continue research on advanced centrifuges, machines that purify uranium for use in nuclear power plants or, if very highly enriched, in weapons, at an underground facility, and the immediate lifting of all UN sanctions and the most severe US and European Union sanctions.
"There has been massive progress on all the issues," a senior Iranian official told Reuters. "There are still disputes over two issues - R&D (research and development) and UN sanctions."
A Western official close to the talks confirmed that from his side, centrifuge research and enrichment in general remained the most difficult unresolved issue: "The essential element for us is R&D, and enrichment."
The United States and European partners are reluctant to allow Iran to operate centrifuges at the Fordo enrichment site, Western officials said, adding that the issue was unresolved.
An Iranian government website said in November that Washington could let Iran keep some 6,000 early-generation centrifuges, down from nearly 10,000 now in operation out of under 20,000 installed.
After meeting US Secretary of State John Kerry on Friday, Iranian Foreign Minister Mohammad Javad Zarif told reporters outside the 19th century hotel on the banks of Lake Geneva where the talks are taking place that it was unclear if there would be a deal in the coming days.
"The negotiations are difficult and complicated and there are highs and lows," Zarif said. "We think an agreement is still possible but when is another story. Our feeling is that we certainly will be able to reach an agreement, but that will need political will on the other side."
Zarif added that the issue of the Saudi-led military operations against Yemen's Houthi fighters, which Tehran has backed, had come up on the sidelines, though the Lausanne talks were exclusively focused on the nuclear issue.
Iranian President Hassan Rouhani spoke with his French, Russian, British and Chinese counterparts on Thursday in an attempt to break the impasse. He also sent a letter to the leaders of all six powers, including US President Barack Obama, though officials said the letter did not suggest Tehran was ready to compromise.
Western officials said the main problem remains Tehran's refusal to offer serious concessions. Iranians say the same thing about the six and accuse the French of taking the hardest line.
They also said there was no guarantee a deal would be clinched by the deadline, but several foreign ministers are due to arrive in Lausanne this weekend.
French Foreign Minister Laurent Fabius, whose country has been making the most stringent demands on Iran according to negotiators, will arrive on Saturday. His British and Russian counterparts have also confirmed they will join the talks.
If there is a political framework agreement in the coming days, the US and European delegations want it to be as specific as possible, including figures for permissible numbers of centrifuges Tehran could operate, uranium stockpiles and other sensitive technical issues.
Further technical details would be included in annexes to be agreed before July 1.
"We are not messing about here," a Western diplomat said. "If there is a deal it won't be a vague understanding that collapses as soon as we leave. If there is a political framework agreement it will have the broad parameters of the issues even if, given the complexities surrounding these talks, it may mean that some issues are not explained in detail."
The six powers want the limits on the most sensitive aspects of Iran's nuclear program to be in place for at least a decade followed by years of intrusive inspections by the UN nuclear watchdog.
They also want to be certain Tehran would need at least one year to produce enough high enriched uranium for a weapon should the Iranians decide to produce one. Iran denies having any nuclear weapons ambitions.
Israel, Saudi Arabia, France and the US Congress have all raised concerns that the Obama administration might be willing to conclude a deal that would allow Iran to develop a nuclear weapons capability in the future.