(Reuters) - The foreign ministers of Iran and six world powers met on Monday in a final push for a preliminary nuclear accord less than two days before their deadline as Tehran showed signs of backing away from previous compromise offers.

For days Iran, the United States, Britain, France, Germany, Russia and China have been seeking to break an impasse in negotiations aimed at stopping Tehran having the capacity to develop a nuclear bomb in exchange for an easing of international sanctions that are crippling its economy.

But officials at the talks in the Swiss city of Lausanne said attempts to reach a framework accord could yet fall apart.

A Western diplomat said there are three major sticking points that must be resolved if Iran and major powers are to secure a framework deal before a self-imposed end-March deadline and it is unclear whether those differences will be bridged.

The diplomat said the most difficult issues were related to the duration of any limits on Iranian enrichment and research and development activities after an initial 10 years, the lifting of U.N. sanctions and restoring them in case of non-compliance by Iran.

"It seems that we have an accord for the first 10 years, but with regard to the Iranians the question of what happens after is complicated," the official said on condition of anonymity, adding: "I can't say what the final result will be."

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

Russian Foreign Minister Sergei Lavrov's spokeswoman said he was returning to Moscow later on Monday though officials said he would return to Switzerland if there was something to announce.

The Russian Foreign Ministry said there had been "considerable progress" and that Lavrov urged all sides to be ready to compromise.

In addition to U.S. Secretary of State John Kerry, Iranian Foreign Minister Mohammad Javad Zarif, Steinmeier and Lavrov, British Foreign Secretary Philip Hammond, French Foreign Minister Laurent Fabius and China's Wang Yi gathered at a 19th-century hotel overlooking Lake Geneva.

The ministers met for an hour and were expected to meet again later on Monday.

Israeli Prime Minister Benjamin Netanyahu, who has campaigned vehemently against the negotiations, said in Jerusalem that the agreement being put together in Lausanne sends the message "that Iran stands to gain by its aggression".

COLD FEET?

Despite deep disagreements on several points, Western officials said the two sides had previously been closing in on a preliminary deal that could be summarized in a brief document which may or may not be released.

Officials said the talks could run at least until the deadline of midnight on Tuesday or beyond. If there was a deal in Lausanne, the parties might move to Geneva for a ceremony.

The six powers 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, demands in exchange for limits on its atomic activities a swift end to sanctions.

Both Iran and the six have floated compromise proposals but agreement has remained elusive.

One sticking point concerns Iran's demand to continue with research into newer generations of advanced centrifuges that can purify uranium faster and in greater quantities than those it currently operates for use in nuclear power plants or, if very highly enriched, in weapons.

Another question involves the speed of removing United Nations sanctions on Iran.

Even if Iran and the six powers reach a framework agreement by their end-March deadline, officials say it could still fall apart when the two sides attempt to agree on all the technical details for a comprehensive accord by June 30.

There were several examples of both progress and setbacks. Western officials said Iran suggested it would could keep fewer than 6,000 centrifuges in operation, down from its current figure of nearly 10,000, and to ship most of its enriched uranium to Russia.

But Iranian negotiator Abbas Araqchi said dispatching stockpiles abroad "was not on Iran's agenda".

A senior U.S. State Department official said there had been no decisions on stockpiles, though several officials made clear that the Iranians had given preliminary consent to the idea before reversing their position. Still, negotiators said stockpiles were not a dealbreaker.

The U.S. official said the goal was to ensure that "the amount of material remaining as enriched material will only be what is necessary for a working stock and no more".

Another Western official said there were "other options" for dealing with the stockpile issue if it was not shipped to Russia, such as downblending to make it less pure.

It was not clear if the Iranian backtracking on certain proposals was a sign that Tehran might be getting cold feet.

On the issue of U.N. sanctions, officials voiced concerns that the five permanent veto-wielding council members could object to plans to strip away some of the U.N. measures in place since 2006, albeit for different reasons.

Britain, France and the United States want any removal of U.N. sanctions to be automatically reversible, but the Russians dislike this because it would weaken their veto power, a Western official said.

By Reuters