Negotiations between Iran and the Western powers over the Islamic republic's nuclear programme have yielded no breakthrough, and major gaps remain, sources have told Al Jazeera.
US Secretary of State John Kerry and his Iranian counterpart met early on Tuesday in the Swiss city of Lausanne as they continued the talks.
A March 31 deadline looms for Kerry and Mohammad Javad Zarif to agree the outlines of an agreement, over the objections of US and Iranian hardliners wary of any deal.
I see the elements for a deal to be reached but I still see the gaps that need to be filled
European Union foreign policy chief Federica Mogherini said that all sides were aware how important it is to seal a good deal and that it was not clear whether "a technical solution" to fill the remaining gaps can be found.
"I see the elements for a deal to be reached but I still see the gaps that need to be filled," Mogherini, who is negotiating with Iran on behalf of the world's five nuclear powers and Germany, said.
"We're still making progress but there is a long way to go if we're going to get there," British Foreign Secretary Philip Hammond said, after talks in Brussels with his French, German, EU and Iranian counterparts.
The talks were aimed at narrowing gaps in the positions between Iran and the world powers, as part of a 15-month negotiating process that could see Iran freeze its nuclear programme for at least a decade in exchange for the gradual lifting of international sanctions.
Iran says the programme is aimed at generating electricity and at medical research, but some in Europe and the US fear it is trying to covertly build atomic weapons.
"It's always useful to talk but we, the French, want a solid deal," said French Foreign Minister Laurent Fabius. "Certain points are yet to be resolved, we hope we will be able to resolve them but as long as it's not done, it's not done."
The world powers and Iran have set an end-of-March deadline to reach a framework accord on the way ahead. Some officials have said persistent differences mean negotiators could settle for an announcement that they've made enough progress to justify further talks.
The meeting between Kerry and Zarif included US Secretary of Energy Ernest Moniz and Iran's nuclear chief Ali Akbar Salehi, who also met on Sunday to negotiate technical details of Iran's nuclear programme.
Senior officials from the world powers will continue negotiations with Iran in Lausanne on Wednesday.
Zarif, who represented Iran in Brussels, will also hold several days of discussions with the US secretary of state in Switzerland this week, as momentum builds in the nuclear negotiations.
Israel and some Western and Gulf Arab countries suspect Iran of ambitions to create an atomic weapon. The Islamic republic denies that accusation.
Reporting from Lausanne, Al Jazeera's diplomatic editor James Bays said: "John Kerry doesn't just have to do serious diplomacy here to get a deal, but if he does get a deal, he has to do serious damage limitation with key actors in all of this."
"The Saudis say if Iran has nuclear technology then Saudi should have nuclear technology and they signed a memorandum of understanding on that very issue with South Korea just a week ago. Other countries in the Gulf are thinking along similar lines."
Under a November 2013 interim deal with the six world powers, Iran stopped expanding its activities in return for minor sanctions relief. The US and Iran have not had diplomatic relations for 35 years but the 2013 election of President Hassan Rouhani resulted in a minor thaw and a diplomatic push to resolve the more than decade-old nuclear standoff.
Since then the parties have been pushing for a lasting accord, amid the alarm of Israel, US Republicans and Gulf Arab states warning the US administration over a bad deal.
Source: Al Jazeera And Agencies