Senior diplomats from the E3 group of Britain, France and Germany expressed "disappointment and concern after thoroughly and carefully analysing Iranian proposed changes to the text negotiated during the previous six rounds", which took place earlier this year.

"Tehran is walking back almost all of the difficult compromises crafted after many months of hard work," they said, adding that the Iranian delegation had demanded "major changes".

They went on to say it was "unclear how these new gaps can be closed in a realistic timeframe".

The latest round of talks began on Monday between the E3, Iran, China and Russia, with the US participating indirectly.

Diplomats were aiming to revive the 2015 deal, which began unravelling in 2018 when US president Donald Trump pulled out of the deal and reimposed sanctions, prompting Iran to start exceeding limits on its nuclear programme the following year.

The diplomats said the delegations needed to "return to capitals to assess the situation and seek instructions, before reconvening next week to see whether gaps can be closed or not".

"Our governments remain fully committed to a diplomatic way forward. But time is running out," they said.