Iran and the US showed little sign Saturday of an early breakthrough in last-ditch nuclear talks with both sides warning of "hard work" ahead and France stressing key issues remain unresolved.
US Secretary of State John Kerry and Iran's Foreign Minister Mohammad Javad Zarif met twice during the day for what is set to be the final push to seal an accord curtailing Tehran's nuclear programme after almost two years of negotiations.
Kerry told reporters that although he remained "hopeful" ahead of Tuesday's deadline "we've a lot of hard work to do".
Zarif agreed negotiators "need to work really hard in order to be able to make progress and move forward".
But his deputy Abbas Araghchi suggested parts of a framework deal reached in April in Lausanne could no longer be applied because other countries had changed their positions.
"In Lausanne we found solutions to many things, but some issues remained unresolved," he told an Arabic-language Iranian television channel Al-Alam.
"And now some of the solutions found in Lausanne no longer work, because after Lausanne certain countries within the P5+1 made declarations... and we see a change in their position which complicates the task."
Officials have acknowledged the June 30 deadline may slip by a few days.
Other foreign ministers from the so-called P5+1 -- Britain, China, France, Germany, Russia and the United States -- are due to arrive in the coming days, with EU foreign policy chief Federica Mogherini expected on Sunday.
French Foreign Minister Laurent Fabius, who flew in Saturday, said at least three "indispensable" demands remained unresolved.
"We want a robust accord that recognises Iran's right to a civilian nuclear programme, but which guarantees that Iran renounces definitively nuclear weapons," he said.
Fabius, who met separately with both Zarif and Kerry, stressed there must be "a lasting limitation of Iran's nuclear capacities in research and production" as well as rigorous inspection of military sites, and a mechanism to quickly reimpose sanctions.
It is hoped a deal would end a standoff dating back to 2002 which has threatened to escalate into war and poisoned the Islamic republic's relations with the outside world.
Any deal faces close scrutiny by hardliners in Iran and the United States, as well as Iran's regional rivals Israel, widely assumed to have nuclear weapons itself, and Saudi Arabia.
According to the Lausanne framework, Iran will slash by more than two-thirds its uranium enrichment centrifuges, which can make fuel for nuclear power or the core of a nuclear bomb, and shrink its uranium stockpile by 98 percent.
Iran also agreed to change a planned reactor at Arak so it cannot produce weapons-grade plutonium and no longer to use its Fordo facility -- built into a mountain to protect it from attack -- for uranium enrichment.
In return it is seeking a lifting of EU, US and UN sanctions which have choked its economy and limited access to world oil markets.
But on Tuesday Iran's supreme leader, Ayatollah Ali Khamenei, set out key "red lines" for the final agreement that appeared to go against the Lausanne deal.
These included the timing of sanctions relief and UN access to military bases, needed to investigate claims of past bomb-making efforts and to probe any future suspicious activity.
Arms Control Association analyst Kelsey Davenport said although the comments were "unhelpful, they are unlikely to derail the talks" stressing Khamenei's position has shifted in the past.
Negotiators cannot extend the talks too long however since under new US legislation Congress must receive a copy of the draft by July 9, otherwise lawmakers will have 60 days to review and vote on the deal instead of 30.