LAUSANNE, Switzerland (Reuters) - Major powers and Iran were closer to a preliminary accord on Tehran's nuclear program as marathon talks ran into Wednesday, but were stuck over key details such as lifting U.N. sanctions and Iran's future atomic research.

After missing a self-imposed March 31 deadline for a deal, the negotiators ended talks in the Swiss city of Lausanne in the early morning hours with an air of chaos, disunity and cacophony as delegations scrambled to get contradictory viewpoints across.

The six powers - the United States, Britain, France, Germany, Russia and China - aim to stop Iran from gaining the capacity to develop a nuclear bomb in exchange for easing international sanctions crippling its economy.

Iran says its nuclear programme is peaceful

France's foreign minister, often seen as making the most stringent demands of Iran, returned to Paris because things had not advanced enough for an "immediate deal".

But as negotiators from the powers met Iranian Foreign Minister Mohammad Javad Zarif again on Wednesday, Russia and Iran expressed optimism an initial agreement was within reach.

British Foreign Secretary Philip Hammond sounded a note of caution. "I think we have a broad framework of understanding, but there are still some key issues that have to be worked through," he told the BBC.

Western diplomats said Iran had on Tuesday reaffirmed its "nuclear rights", suggesting the talks were again getting bogged down entering their seventh day.

Officials cautioned that any agreement would probably be fragile and incomplete.

"We hope to wrap up the talks by Wednesday night ... We insist on lifting of financial, oil and banking sanctions immediately ... for other sanctions we need to find a framework," senior Iranian negotiator Abbas Araqchi told state television. "We insist on keeping research and development with advanced centrifuges," he said.

Araqchi said he expected the parties to issue a joint statement declaring that "progress has been made in the talks and that we have come to a solution on key issues. We will have the solutions in written form."

U.S. THREAT

German Chancellor Angela Merkel told reporters in Berlin she hoped for a compromise ensuring Iran cannot get nuclear weapons. All sides had made a lot of progress, she said, but "such negotiations only come to an end if there is agreement on all points."

Other diplomats close to the talks said any preliminary deal would include a document with some key figures - such as permitted numbers for centrifuges and uranium stockpiles - though it would remain confidential for the foreseeable future.

A preliminary deal would be a major milestone towards a final accord, with an end-June deadline, that could end a 12-year standoff between Iran and world powers and reduce the risk of another Middle East war.

But it would only be a first step and reaching agreement on details by June 30 will be difficult.

The United States threatened on Tuesday to walk away if the talks yield no political framework accord.

The talks have stalled on the issues of Iran's nuclear centrifuge research, the lifting of U.N. sanctions and their restoration if Iran breaches the agreement.

RUSSIA SAYS KEY DETAILS AGREED

Russian Foreign Minister Sergei Lavrov, who returned to Moscow, said negotiators had reached a general accord on "all key aspects."

But a diplomat close to the talks denied such an agreement had been reached, and French Foreign Minister Laurent Fabius left saying he would only return if it was "necessary".

"Things have progressed, but not enough ... that we can reach an immediate deal," Fabius said in Paris. "We are firm. We want a robust deal with detailed checks."

China warned of failure and urged all sides to compromise.

"If the talks are stuck, then all previous efforts to resolve Iran's nuclear standoff with the six major powers will have been wasted," said the rare statement by the Chinese delegation after Foreign Minister Wang Yi left Lausanne.

The U.S. administration of President Barack Obama had committed to meeting an end-March deadline for the outline political accord. Diplomats said they knew U.S. credibility was at stake after missing that deadline and getting a deal in the coming hours was crucial.

Some parts of any understanding reached by the parties will likely remain confidential.

Congress has warned it will consider imposing new sanctions on Iran if there is no agreement this week. A Democrat, Obama has threatened to veto any such sanctions moves.

Lavrov said the possible agreement included the International Atomic Energy Agency's monitoring of Iran's nuclear programme as well as steps to lift sanctions. Experts would work out technical details by end-June.
"One can say with enough confidence that ministers have reached a general agreement on all key aspects of a final settlement to this issue," Lavrov said.


A senior Iranian negotiator said Tehran was willing to negotiate until the deadlock was resolved. "Iran does not want a nuclear deal just for the sake of having a deal, and a final deal should guarantee the Iranian nation's nuclear rights," the negotiator, Hamid Baidinejad, told reporters.

By Reuters