France's foreign minister is joining talks on Iran's nuclear programme in Switzerland and has said he wants to achieve a "robust" deal.

Laurent Fabius joined US Secretary of State John Kerry and Iranian foreign minister Mohammad Javad Zarif ahead of a Tuesday deadline to agree a historic agreement that could see Iran scaling back its nuclear programme in exchange for sanctions relief.

"I am coming here with the desire to move towards a robust agreement," Fabius told reporters in Lausanne. "We have made progress on certain issues but not enough on others."

He said he was "insisting" that any deal included mechanisms to ensure that the Islamic Republic complies with its commitments.

German Foreign Minister Frank Walter Steinmeier was due to join his counterparts later on Saturday and hold a working lunch with Kerry and Fabius, US officials said.

Russia's Foreign Minister Sergey Lavrov confirmed plans to visit Lausanne on Sunday for the negotiations, Deputy Foreign Minister Sergey Ryabkov told reporters on Saturday.

"Lavrov plans to arrive on Sunday to attend the talks on Iran's nuclear programme," said Rybkov, who is Russia's representative at the talks. China's foreign minister was also expected to attend on Sunday.

'Toughest part'

Al Jazeera's James Bays, reporting from Lausanne, said the negotiators had reached an "end phase" of the talks and were working hard to beat the deadline.

"We're at this stage where we are very close to the deadline, you are going to see talks all day and all night ... the last part of this negotiation is the toughest part, the deadline at the end of the month," Bays said.

Six world powers - the five permanent members of the UN Security Council plus Germany - have been involved in the negotiations seeking to agree a deal by March 31 that could pave the way for a broader and definitive agreement on how Tehran's nuclear programme should proceed.

France is said to have been particulalrly tough and uncompromising at the negotiating table, reportedly demanding more stringent restrictions than the other delegations.

The West suspects Iran's nuclear programme is intended for military purposes, but Tehran says it is meant to generate power.