With Monday’s deadline looming to reach a deal on Iran’s nuclear programme, an extension to the talks is looking likely.
US negotiators in Vienna remain optimistic about the possibility of an agreement despite big gaps which they are said to be chipping away at.
The six world powers say Iran still refuses to curb uranium enrichment in return for having sanctions lifted. Tehran rejects Western suspicions it is developing a nuclear weapon.
Iran is facing domestic pressure not to make concessions.
The supreme leader Ayatollah Ali Khamenei has the final say on a deal, and has repeatedly warned that he is pessimistic over the chances of reaching one.
The Russian Foreign Minister is expected to join talks this evening as the talks head into their final hours.