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Talks on Iran's nuclear programme have entered a critical phase in Switzerland, with negotiators making last-ditch efforts to reach a preliminary deal before the March 31 deadline.

Top diplomats from Britain, China, France, Russia, the US and Germany - collectively known as P5+1 - are meeting with Iranian officials in Lausanne to try to bridge remaining gaps and hammer out the framework deal that would be the basis for a final accord to be reached by the end of June.

Russia's Foreign Minister Sergei Lavrov, who left the talks on Monday, returned to Lausanne on Tuesday to join the discussions.

"Probably if there is a realistic chance of a deal tomorrow [Tuesday] he will come back," foreign ministry spokeswoman Maria Zakharova had told reporters in Lausanne on Monday.

Analysis from our correspondent
Al Jazeera's James Bays

All of the people needed to sign a deal with Iran are now in Lausanne, and the negotiations are closer than ever before. 

Many of the main technical issues concerning Iran's uranium and plutonium facilities appear to have been dealt with. 
However, there are still sticking points, especially how much future research and development Iran would be allowed to carry out to further advance its nuclear programme, beyond the initial 10 year deal proposed by the US.
 
Iran wants all sanctions, particularly those imposed by the UN, to be lifted, but the other nations want them to continue in some form.
 
It is yet to be determined how exactly the deal will be announced; how much will be in a formal document and how much will be shared publicly.
 
But what is clear is that no deal will be done without all foreign ministers present. A US official tells us that the talks will not go beyond the 31st of March.
 
The weather in Lausanne is cold and wet – a sign of good luck in Persian culture. Nonetheless, difficult negotiations are still likely.

For weeks, US Secretary of State John Kerry and his Iranian counterpart, Mohammad Javad Zarif, have been meeting in an intense effort to reach a political understanding on terms that would curb Iran's nuclear activities in exchange for sanctions relief.

"We're working hard, very hard", Kerry told Al Jazeera on Monday. 

Kerry's spokeswoman Marie Harf said there was a 50/50 chance the parties would succeed in getting a deal on Tuesday.

Senior Iranian negotiator Abbas Araqchi said various proposals were still being discussed.

"Still, we cannot say we are close to resolving the remaining issues," he told reporters on Monday.

"We hope to reach a conclusion tonight or tomorrow, but it is not guaranteed and we have a difficult way to go."

Meanwhile, a senior State Department official told the Reuters news agency that the US was willing to continue working past the deadline if it appeared that progress was being made.

"Our team is evaluating where we are throughout the day and making decisions about the best path forward," the official said.

"We will of course keep working if we are continuing to make progress, including into tomorrow [Wednesday] it it's useful to do so."

Officials earlier said the sides have made some progress, with Iran considering demands for further cuts to its uranium enrichment programme but pushing back on how long it must limit technology it could use to make atomic arms. 

Sticking points

But sticking points on research and development remain, as well as differences on the timing and scope of sanctions removal, officials cited by the Associated Press news agency said.
 

The officials in Lausanne said the sides were advancing on limits to aspects of Iran's programme to enrich uranium, which can be used to make the core of a nuclear warhead.   

Tehran has said it is willing to address concerns about its stockpiles of enriched uranium, although it has denied that will involve shipping it out of the country, as some western officials have said. One official said on Monday that Iran might deal with the issue by diluting its stocks to a level that would not be weapons grade.  

A senior State Department official said that shipping the stockpile is one of the "viable options that have been under discussion for months ... but resolution is still being discussed."

Meanwhile, Israel, which is at loggerheads with Iran, said the deal being negotiated was worse than it feared.

"I expressed our deep concern towards this deal emerging with Iran in the nuclear talks, this deal as it appears to be emerging bears out all of our fears, and even more than that," said Prime Minister Benjamin Netanyahu.

Israel and the US insist Tehran is seeking to build a nuclear bomb, but Tehran says its nuclear programme is intended for power generation.

Source: Al Jazeera and agencies