"The negotiations are difficult. They've been difficult since the beginning and they still are," Iranian Foreign Minister Mohammad Javad Zarif told AFP on Friday after meeting in Lausanne with top US diplomat John Kerry for about 90 minutes.
Another member of the Iranian delegation said: "It all depends on the willingness of the other side. All the solutions are on the table."
Iranian President Hassan Rouhani made an extraordinary appeal to world leaders on Thursday writing a letter to US President Barack Obama, even though their countries do not have diplomatic ties, and in a blitz phoning 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 said.
US officials have confirmed that a letter to Obama was passed to Kerry and his team by the Iranians, but have refused to comment on its content.
The negotiations in Lausanne are aimed at agreeing by Tuesday the contours of a deal that world powers hope will thwart any Iranian drive to develop nuclear weapons.
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 annexes by June 30.
World powers hope to scale back Iran's nuclear capacity by cutting its number of sophisticated centrifuges for spinning enriched uranium, and reconfiguring some of its facilities, such as the underground Fordo plant.
"Our goal is, as it's always been, to have Fordo converted so it's not being used to enrich uranium," a senior US official told AFP Friday, refusing to go into further details of what was being negotiated.
Kerry needs to return to Washington with something concrete to head off threatened fresh US sanctions by the opposition Republicans, who together with Israel fear the mooted deal will be too weak.
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.
Foreign ministers gathering
"Everyone wants to reach a deal," said another diplomat, asking not to be named. "Everyone has invested so much in the past year, so much work has been done."
Britain's foreign office confirmed Foreign Secretary Philip Hammond would arrive in Lausanne this weekend, joining France's hawkish Laurent Fabius who is expected early Saturday, according to Paris.
It was possible that German Foreign Minister Frank-Walter Steinmeier may also "travel to Lausanne, but no decision has been made yet," his spokesman said.
Highlighting one of the main difficulties in the talks, Rouhani, whose 2013 election led to the current diplomatic push, also called Thursday 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.
In a sign of the international diplomacy gathering pace, Russian President Vladimir Putin spoke by phone with Israeli Prime Minister Benjamin Netanyahu during which they raised "problems linked to the Iranian nuclear programme" as well as other global crises.
Russian Foreign Minister Sergei Lavrov could arrive in Lausanne on Sunday evening, the Russian news agency RIA Novosti said, citing anonymous sources.
It was uncertain whether his Chinese counterpart would also arrive.
There were concerns that a crisis in Yemen, where Saudi Arabia has launched air strikes against Shiite rebels who have seized control of the capital, could sour the atmosphere in Lausanne.
Although the issue was raised by Kerry with Zarif, US officials say the crisis has had "no impact" on the talks.