Marathon Iran nuclear talks headed on Friday towards a critical weekend as Britain said its foreign minister would join his US, 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 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 told Iranian media.
"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.
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.
But both Iran and France have criticised the two-step process, with France's US ambassador calling it a "bad tactic".
"Sometimes the differences of opinion on the other side make for contradictory positions," Zarif said Friday.
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.
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 air strikes 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 on 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.
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