Iran nuclear talks continue as Tuesday deadline looms

 officials have been negotiating with representatives from six world powers on Iran's nuclear programme
Talks on Iran's nuclear programme have entered a critical phase, with negotiators making last-ditch efforts to reach a preliminary deal before the March 31 deadline.
Top diplomats from Britain, China, France, Russia, the United States and Germany – collectively known as P5 1 - are meeting with Iranian officials in the Swiss city of 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.
But Russia's Foreign Minister Sergei Lavrov said on Monday he was leaving the talks and would return the following day if there is a realistic chance of a deal, a spokeswoman said, raising doubts on chances of reaching an agreement on Monday.
"Probably if there is a realistic chance of a deal tomorrow [Tuesday] he will come back," foreign ministry spokeswoman Maria Zakharova told reporters in Lausanne.

Analysis from our correspondent

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."
Officials say 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.
 

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.