By Louis Charbonneau and John Irish
LAUSANNE, Switzerland (Reuters) - The United States and Iran resumed negotiations on Thursday aimed at clinching a nuclear deal before a March 31 deadline, and officials close to the talks said some kind of preliminary agreement between Tehran and six powers was possible.
As the talks began, Washington and Tehran took opposing stands on Saudi-led air strikes in Yemen against rebels allied to Iran who are fighting to oust the country's president, but it was unclear whether this would affect the nuclear talks.
The goal of the negotiations, underway for nearly 18 months, is an accord under which Iran would halt sensitive nuclear work for at least a decade. In exchange, international sanctions on Iran would be lifted under the deal to end its 12-year nuclear standoff with the West and reduce the risk of another war in the Middle East.
U.S. Secretary of State John Kerry and Energy Secretary Ernest Moniz met their Iranian counterparts, Foreign Minister Mohammad Javad Zarif and Atomic Energy Organization chief Ali Akbar Salehi in the Swiss city of Lausanne.
Earlier, Iranian media quoted Zarif as condemning the Saudi-led military operation against the Shi'ite Muslim Houthi fighters in Yemen, and demanding that it stop.
By contrast, Kerry spoke to the foreign ministers of Saudi Arabia and other Gulf Cooperation Council members on Thursday and welcomed their decision to take action against the Houthis, a senior U.S. official said.
However, neither Kerry nor Zarif responded when asked by a reporter in Lausanne to comment on the air strikes.
Speaking to reporters traveling with Kerry from Washington on Wednesday, a senior State Department official said the six powers - the United States, Britain, France, Germany, Russia and China - would not rush to complete a framework agreement with Iran just because there was a March 31 deadline.
But the official said the parties had made progress at last week's inconclusive round of negotiations in Lausanne.
"We very much believe we can get this done by the 31st," the official said. "We see a path to do that." The official added, however, that there was no guarantee of success.
Israel, Saudi Arabia, France and U.S. Congress have all raised concerns that the administration of President Barack Obama might be willing to conclude a deal that would allow Iran to develop a nuclear weapons capability in the future.
The official said: "Any political understanding needs to address in some way all of the elements of a final agreement."
"We do not know what form this will take ... We have always said it needs to have specifics. We will need to communicate as many specifics as possible in some form or fashion (to the public and U.S. Congress)."
Those elements include the different ways to a nuclear weapon, ensuring that it would take Iran at least one year to produce enough high enriched uranium for a single bomb, research and development into advanced centrifuges, transparency measures and monitoring, and sanctions relief for Iran.
FABIUS TO JOIN TALKS
Iran's supreme leader, Ayatollah Ali Khamenei, opposes the idea of a two-step process. Iranian officials say they fear a written framework accord would curtail Tehran's negotiating space for a final deal by the end of June.
Iranian officials have also suggested they could accept some kind of statement or political declaration in Lausanne, as opposed to a formal written agreement.
Officials close to the talks said deep disagreements remained between Tehran and the powers, while divisions had also emerged in recent weeks between the United States and France on what to demand of Tehran.
French Foreign Minister Laurent Fabius will go to Lausanne on Saturday to join the talks, a ministry spokesman said.
Iran denies Western allegations it is seeking the capability to procure atomic weapons. But Israel, which is believed to have the Middle East's only nuclear arsenal, has previously threatened Iran with military attack.
With the Republican-led U.S. Congress threatening to vote on new sanctions against Iran if there is no agreement this month, the Obama administration is pushing hard to secure a deal. Obama has vowed to veto any new sanctions moves.
Other officials said some kind of agreement in Lausanne was possible. "The aim is to get a sort of memorandum of understanding that would be enough for Americans to take to Congress and the Iranians to keep to Khamenei's demand," said a Western diplomat involved in the talks.
"The aim is to get something out by Sunday, although the deadline is March 31," the official added.
The main obstacle, Western officials say, remains Iran's refusal to compromise on sanctions, research and development and other issues.
Even if there is a deal in the next few days, the U.S. official said much work will remain between now and the end of June to work out technical details. And there was no guarantee the talks would not collapse later.
The six powers and Iran have twice extended talks on a long term accord after missing deadlines in the wake of an interim agreement struck in 2013.
(additional reporting by Parisa Hafezi; editing by Andrew Hay and David Stamp)