LAUSANNE: Marathon�Iran�nuclear talks headed Friday towards a critical weekend as�Britain�said its foreign minister would join his U.S., Iranian and French counterparts in racing the clock to agree the contours of a deal.

"The negotiations are very tough and complicated and there are highs and lows," Iranian Foreign Minister�Mohammad Javad Zarifsaid Friday before going into talks in Switzerland with top U.S. 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 told Iranian media.

This followed an extraordinary appeal by Iran's president to world leaders Thursday that saw him write a letter to U.S. President Barack Obama and phone his counterparts in Britain, China,�Franceand 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 said.

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 U.S. sanctions by the opposition Republicans, who together with Israel fear the mooted deal will be too weak.

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

A Western diplomat involved in the talks said Thursday that something vague and "wishy-washy" at the end of this round would not be sufficient.

"Sometimes the differences of opinion on the other side make for contradictory positions," Zarif said Friday.

Britain's foreign office confirmed Foreign Secretary�Philip Hammond�would arrive in Lausanne this weekend, joining France's hawkish Laurent Fabius who is expected Saturday morning, according to Paris.

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 Sunday evening.

The content of his letter to Obama, confirmed by Washington, was not known, but Rouhani's official Twitter account quoted him as saying that a deal would be "beneficial to region & world."

But highlighting one of the main difficulties in the talks, Rouhani, whose 2013 election led to the current diplomatic push, also called for "unjust" sanctions choking the country's economy to be lifted.

"The peaceful character of [Iran's] nuclear activities and the necessity to annul all the unjust sanctions can lead us to a final deal," Rouhani's office quoted him as telling Cameron.

The six powers are however insisting that sanctions will only be suspended, not lifted, to enable them to be quickly put back in place if Tehran violates the deal.

The�US Senate�voted unanimously Thursday supporting a non-binding measure to slap new economic sanctions on Iran should it violate terms of any nuclear deal reached.

There were concerns meanwhile that a crisis in Yemen, where Saudi Arabia launched airstrikes against Shiite rebels who have seized control of the capital, could sour the atmosphere in Lausanne.

Zarif slammed the military action, in which health officials said Friday have so far killed at least 39 civilians, while Kerry "commended" them.

A State Department spokesman,�Jeff Rathke�said Thursday that Kerry "did briefly raise Yemen with his Iranian counterpart. But let me stress, this was not and is not the focus of the talks."

And another US official insisted the air strikes would have "no impact" on the nuclear talks.

By AFP