By Louis Charbonneau, John Irish and Parisa Hafezi

LAUSANNE, Switzerland (Reuters) - Iran and six world powers tried to break an impasse in nuclear negotiations on Sunday, but officials cautioned that attempts to reach a preliminary deal by a deadline in two days could yet fall apart.

The two sides explored compromises in areas including numbers of centrifuges used to enrich uranium that Iran could operate, and its nuclear enrichment work for medical research.

But Israel, which feels especially threatened by the possibility of a nuclear-armed Iran, said details of a possible framework agreement emerging from the talks in Lausanne, Switzerland, were even worse than it feared.

Foreign ministers from the six countries will hold ‎the first full meeting with Iran's foreign minister on Monday morning.

German Foreign Minister Frank-Walter Steinmeier said there had been "some progress and some setbacks in the last hours".

"I can't rule out that there will be further crises in these negotiations," he told reporters in Lausanne.

The United States, Britain, France, Germany, Russia and China want more than a 10-year suspension of Iran's most sensitive nuclear work. Tehran, which denies it is trying to develop a nuclear weapons capability, is demanding an immediate end to international sanctions that are crippling its economy.

A Western diplomat said duration could be traded off if there were real efforts on some key parameters.

“We all want it to be 15 years, but there will be different durations for various aspects of the deal," the diplomat told reporters.

Iranian negotiator Hamid Baidinejad said "15 years is out of question for Iran but 10 years is being discussed".

Officials warned that deep disagreements remained on several points but said the two sides had been closing in on a preliminary deal that could be summarized in a brief document which may or may not be released.

Several officials told Reuters that Tehran had indicated a willingness to cut the number of centrifuges it uses to fewer than 6,000, thereby slowing its program, and to send most of its enriched uranium stockpiles for storage in Russia.

Senior Iranian negotiator Abbas Araqchi told reporters dispatching stockpiles abroad "was not on Iran's agenda".

Western powers were meanwhile considering allowing Iran to conduct limited and closely monitored enrichment-related work for medical purposes at an underground facility, the officials added on condition of anonymity.

Iran had originally insisted on keeping in operation all the nearly 10,000 centrifuges it currently uses, but said in November that Washington had indicated it could accept around 6,000. Iranian officials say they have been pushing for 6,500-7,000.

Baidinejad said cutting the number of Iran's centrifuges to 6,000 "was one of the proposed ideas by the other party".

All parts of an emerging nuclear deal are interrelated. "Everything could still fall apart" before Tuesday's self-imposed deadline for a framework agreement, a Western official told Reuters.

Araqchi said he believed a deal was possible but that serious decisions remained to be taken.

One concerns Iran's demand to continue with research into a new generation of advanced centrifuges that can purify uranium faster and in greater quantities for use in nuclear power plants or, if very highly enriched, in weapons.

Another question is over the speed of removing United Nations sanctions on Iran. A senior U.S. official said there were other unresolved questions but expected those would fall into place if the big sticking points could be worked out.

The U.S. official said negotiators were working towards something that would be called an "understanding" rather than a formal agreement, which would form the basis of a comprehensive deal, including all technical details, to be tied up by June 30.

ISRAEL FURIOUS

A senior European diplomat said ministers were engaged in a "a political push to convince Iran" before Tuesday's deadline, adding: "All the pieces of a possible accord are there. We have to try and put them in place so that everything adds up."

The powers' aim is to ensure that for the next decade Iran is kept at least one year away from being able to produce enough fissile nuclear material for a single weapon.

"It has to be a deal which puts the bomb beyond Iran's reach. There can't be any compromise about that," British Foreign Secretary Philip Hammond said.

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