Lausanne: Iran and six major powers were exploring possible compromises to break an impasse in nuclear negotiations on Sunday, but officials cautioned they were unable to move on several sticking points.
The news came as Israel said the details of a possible agreement emerging from talks in Lausanne, Switzerland were worse than it feared.
In a significant development in talks aimed at securing a preliminary nuclear deal, several officials told Reuters Tehran had indicated a willingness to accept fewer than 6,000 nuclear centrifuges and to send most of its enriched uranium stockpiles for storage in Russia.
Western powers, on the other hand, were considering the idea of allowing Iran to conduct limited, closely-monitored enrichment-related work for medical purposes at an underground facility called Fordow, the officials added on condition of anonymity.
Iran had originally insisted on keeping in operation the nearly 10,000 centrifuges it currently uses, but said in November that Washington indicated it could accept around 6,000. Iranian officials say they had been pushing for 6,500-7,000.
The officials said all parts of an emerging nuclear deal were interrelated.
"Everything could still fall apart," a Western official told Reuters, adding that the talks could drag on to Tuesday, the self-imposed deadline for a framework agreement.
Another official said it was "more or less reality", but that nothing was set in stone yet.
A main sticking point is Iran`s demand that it continue with research into advanced centrifuges. The machines can purify uranium for use in nuclear power plants or, if very highly enriched, in weapons.
Another unresolved question is over the speed of removing some of the United Nations sanctions on Tehran.
The six world powers negotiating with Iran, the United States, Britain, France, Germany, Russia and China, are seeking at least a 10-year suspension of its most sensitive nuclear work. The dispute dates back more than a decade and has fueled fears of a new Middle East war.
Iran denies charges from the West and Israel that it is seeking to develop a nuclear weapons capability, and says its programme is purely peaceful. It demands the removal of international sanctions that are hurting its economy.
The powers` aim is to ensure that Iran is kept at least one year away from producing enough fissile nuclear material for a single weapon for at least 10 years.
The six powers have an end-March deadline for a political framework agreement, which officials close to the talks say could result in a brief document of several pages. That would form the basis of a comprehensive deal, including all technical details, to be agreed by June 30.
Officials say there is no guarantee they will get a framework agreement this month or, if they do, a final one in June.
Top diplomats from the United States, France and Germany cancelled travel plans for the next few days to push for the accord that would lay the foundations for a final settlement.
U.S. Secretary of State John Kerry`s spokeswoman said he would not fly to Boston for a ceremony in honour the late Senator Edward Kennedy, who was his mentor.
Officials close to the talks in the Swiss city of Lausanne also confirmed the French and German foreign ministers, Laurent Fabius and Frank-Walter Steinmeier, had cancelled a planned joint trip to Kazakhstan.
"We`re hopeful, but there is still a lot of work to be done," Fabius told reporters on Sunday.
The remarks from Fabius contrasted with a hostile blast from Israel, which is not a party to the talks.
"This deal, as it appears to be emerging, bears out all of our fears, and even more than that," Prime Minister Benjamin Netanyahu told his cabinet in Jerusalem.
Israel, which is believed to have the Middle East`s only nuclear arsenal, feels especially threatened by the possibility of a nuclear-armed Iran.
Netanyahu referred to advances made by Houthi rebels allied to Tehran in Yemen, and accused the Islamic republic of trying to "conquer the entire Middle East".
"The Iran-Lausanne-Yemen axis is very dangerous to humanity, and must be stopped," he said.
In the past, Israel has threatened to attack Iran if it is not happy with an eventual deal and has long described France as the negotiating power with a position closest to its own, a view confirmed by officials close to the talks.
Kerry and Iranian Foreign Minister Mohammad Javad Zarif have been in Lausanne for days to try to reach a preliminary deal by a self-imposed deadline of Tuesday. They held several rounds of talks on Saturday and met again on Sunday morning.
Fabius and Steinmeier joined the talks on Saturday and Chinese Foreign Minister Wang Yi arrived on Sunday. Their British and Russian counterparts are expected to arrive later.