US Secretary of State John Kerry has landed in Vienna to join negotiations on Iran’s nuclear programme as they enter their final week.
Talks have been continuing between Iran and the P5+1 group, with a deadline of 20 July for an agreement.
An interim deal was reached last year but differences remain between the two sides.
World powers suspect Iran is seeking nuclear weapons – a charge Iran strongly denies.
Iran insists it is enriching uranium to fuel power plants and for medical needs, and wants international sanctions to end.
Differences remain over how much uranium enrichment capacity Iran should be allowed to keep, as well as other issues.
Iranian officials have suggested that the deadline could be extended for another six months to allow negotiations to continue.
Earlier this week, Iran’s Supreme Leader Ayatollah Ali Khamenei has said that Iran could eventually need tens of thousands of centrifuges for its nuclear programme.
Within five years, he said, Iran would need far more than the 10,000 that world powers wanted to limit it to.
In his comments on Tuesday, Ayatollah Khamenei also expressed strong support for Iran’s negotiating team in Vienna, a boost to moderate President Rouhani, who has been accused by hardliners of giving to much ground on the nuclear issue.
Also this week, France’s foreign minister has hinted at growing differences between the Western countries in the talks.
Laurent Fabius said the P5+1 group – the US, France, Britain, Russia, China and Germany – had previously been “very homogeneous” in their dealings with Iran.
But “in recent days representatives in the negotiations have put forward a certain number of different approaches”, he told a parliamentary committee in Paris on Monday.
Both sides are hoping to build on an interim deal agreed in Geneva in November that saw Iran curb enrichment in return for sanctions relief.
But progress since the Geneva deal has been slow, with the last round of talks in May proving unfruitful.