"A pause in #ViennaTalks is needed, due to external factors," the EU's foreign policy chief Josep Borrell tweeted, adding that "a final text is essentially ready and on the table".

The current round of negotiations started in late November between Britain, China, France, Germany, Iran and Russia, with the US taking part indirectly.

Their aim was to revive the 2015 Joint Comprehensive Plan of Action (JCPOA), which began unravelling when former US President Donald Trump withdrew from the deal in 2018.

Last week Russia said it was demanding guarantees that the Western sanctions imposed on its economy following its invasion of Ukraine would not affect its trade with Iran.

As with the original JCPOA in 2015, Moscow had been expected to play a role in the implementation of any fresh deal, for example by receiving shipments of enriched uranium from Iran.

Borrell added on Friday that he would "continue to be in touch with all #JCPOA participants and the U.S. to overcome the current situation and to close the agreement".