The 2015 agreement to curb Iran's nuclear programme in exchange for sanctions relief has been left hanging by a thread since the US withdrew from the pact in 2018 and re-imposed sanctions, prompting Tehran to in turn step up its nuclear activities.

Diplomats from the parties to the deal -- Iran, Britain, China, France, Germany and China -- have been meeting in Vienna since early this month to find a way to get the pact back on track with US participation under the new Joe Biden administration.

"Summing up the results of two weeks of deliberations on JCPOA restoration we can note with satisfaction that the negotiations entered the drafting stage," Russian ambassador to Vienna Mikhail Ulyanov wrote on Twitter, referring to the acronym of the deal's formal name.

"Practical solutions are still far away, but we have moved from general words to agreeing on specific steps towards the goal," he added.

The EU, Russia and Iran all hailed progress at the talks Saturday following an attack on the Natanz nuclear facility, which Iran blamed on arch-foe Israel.

On Friday, Tehran also announced that it was producing uranium enriched to 60 percent purity, taking the country closer to the 90-percent level required for use in a nuclear weapon and far above the threshold allowed by the deal.

Iran has said it will reverse steps taken so far if the US lifts sanctions imposed under the administration of former president Donald Trump.

US National Security Advisor Jake Sullivan told Fox News on Sunday that the US wanted to be sure of Iran's compliance.

"The United States is not going to lift sanctions, unless we have clarity and confidence that Iran will fully return to compliance with its obligations under the deal that it will put a lid on its nuclear programme," he said.

Iran delegation head Abbas Araghchi said Saturday that "a new agreement is taking shape" but warned that it won't be easy.

"We think that negotiations have reached a stage that the parties can start working on a joint text. The writing of the text can start, at least in the fields with a consensus," he said.

"There are still serious disagreements that must be reduced during future negotiations," he added.