U.S. Secretary of Energy Ernest Moniz, U.S. Secretary of State John Kerry and U.S. Under Secretary for Political Affairs Wendy Sherman (L-3rd L) meet with Iranian Foreign Minister Mohammad Javad Zarif (2nd R) at a hotel in Vienna, Austria June 28, 2015. (Photo: Reuters)
Talks between Iran and the US enter a "critical phase" Sunday with tensions rising just three days from a deadline to nail down a deal thwarting any Iranian nuclear arms drive.
EU foreign policy chief Federica Mogherini was due to arrive during the day after US Secretary of State John Kerry and his Iranian counterpart appeared to have made little headway when they returned to the negotiating table in Vienna on Saturday.
"Obviously we are at a critical stage now," a Western diplomat said. "It's become more tense in the final days. But that was always likely to happen."
Global powers known as the P5+1 group -- Britain, China, France, Germany, Russia and the United States -- are seeking to flesh out the final details of a historic accord to curtail Iran's nuclear programme.
They are seeking an accord by Tuesday's June 30 deadline, building on guidelines set by a framework deal agreed in Lausanne on April 2.
Kerry told reporters that although he remained "hopeful" there was still "a lot of hard work to do".
Iranian Foreign Minister Mohammad Javad Zarif agreed, saying 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 the Lausanne framework no longer 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, but several diplomats have categorically ruled out any further formal months-long extension of the talks which have dragged for almost two years now.
With diplomatic pressure growing, other ministers from Britain, China, Germany and Russia are due to follow Mogherini to Vienna over the coming days.
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.
But any deal must stand up to intense 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 a complicated web 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 a final agreement that appeared to go against Lausanne.
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.