World powers are struggling to reach a provisional deal with Iran to limit its nuclear programme.
As talks in Geneva moved into a third day, Iran insisted it would not sign an agreement that did not have specific guarantees of its right to enrich uranium.
Iran’s deputy foreign minister, Abbas Araqchi, said: “We are working on narrowing the gaps but the gaps are still there.”
Enriched uranium is a critical component both in nuclear power generation and nuclear weapons.
Iran insists its nuclear programme is for peaceful purposes but world powers suspect it is seeking a nuclear weapons capability.
Prime Minister Benjamin Netanyahu said: “They must not have nuclear weapons. And I promise you that they will not have nuclear weapons."
Before the latest round of talks opened, Iran's supreme leader Ayatollah Khamenei said Iran would not step back "one iota" from its nuclear rights and referred to Israel as a "rabid dog".
The talks, which are scheduled to conclude today, involve Iran and representatives of the P5+1 - the US, UK, France, China, Russia, and Germany - are now expected to continue into to weekend.
Some US politicians have said they will push for further sanctions against Iran next month if the talks fail.