"The negotiations are very tough and complicated and there are highs and lows," Iranian Foreign Minister Mohammad Javad Zarif said on Friday before going into talks in Switzerland with top US diplomat John Kerry.

"Our feeling is that we can definitely reach a deal but that depends on the political will of the other side," he added.

This followed an extraordinary appeal by Iran's President to world leaders Thursday that saw him write a letter to US President Barack Obama and phone his counterparts in Britain, China, France and Russia.

"We are acting in the national and international interest and we should not lose this exceptional opportunity," Rouhani told British Prime Minister David Cameron, the presidency confirmed.

The negotiations in Lausanne are aimed at agreeing by Tuesday the main outlines of a deal that world powers hope will make an Iranian drive to develop nuclear weapons all but impossible.

A full deal, capping more than a decade of tensions over Iran's atomic ambitions and a year and a half of intense negotiations from New York to Vienna to Oman, is then meant to be rounded out with complex technical agreements by June 30.

Kerry needs to return to Washington with something concrete in order to head off a push for fresh US sanctions by the opposition Republicans, who together with Israel fear the mooted deal will be too weak.

But both Iran and France have criticised the two-step process, with France's US ambassador calling it a "bad tactic".

It was unclear whether the German, Chinese and Russian foreign ministers would also fly in. Russian news agency RIA Novosti cited unnamed sources as saying Sergei Lavrov would arrive on Sunday evening.

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