Negotiations over Iran's nuclear programme have picked up pace with the foreign ministers of France and Germany joining US Secretary of State John Kerry in talks with Iran's top diplomat ahead of a looming deadline for a preliminary deal.

With just four days to go until that target, negotiators in the Swiss town of Lausanne settled in for another round of lengthy sessions that they hope will produce a two-to-three-page outline of an agreement that can then become the basis for a comprehensive deal to be reached by the end of June.

"We're expecting an evening today," Mr Kerry told reporters when asked if he was expecting a good day at the start of his first meeting with Iranian Foreign Minister Mohammad Javad Zarif.

Mr Kerry was to meet later with French Foreign Minister Laurent Fabius and his German counterpart Frank-Walter Steinmeier. The foreign ministers of Russia, China and Britain also are expected in Lausanne over the weekend.

Diplomats at the talks say, however, that their presence does not necessarily mean that a deal is almost done. They say some obstacles remain and the ministers are coming for consultations.

"The talks have been long and difficult. We've advanced on certain issues, not yet enough on others," Mr Fabius said as he arrived.

Iranian nuclear agency chief Ali Akbar Salehi described one or two issues as becoming "twisted," telling Iran's ISNA news agency that the sides were working to resolve the difficulties.

Iran says its nuclear ambitions are purely peaceful, but other nations fear it is seeking to develop weapons.

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