"If the Russian block is confirmed to be definitive, we will be obliged to look at other options," said the diplomat, who asked not to be named, saying the parties did not want to be left in a situation where Russia is "taking the deal hostage".

The EU on Friday announced a pause in the talks -- despite the final text of the deal essentially being ready.

It came after 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.

The diplomat said that the essential issues in the negotiations between world powers and Iran had been concluded when Russia issued its demands last week through a statement by Foreign Minister Sergei Lavrov.

"A failure of this deal... would be extremely damaging and it would be irresponsible for Russia to do so," said the diplomat from the so-called E3 of European powers negotiating the deal.

The diplomat confirmed that the talks had been paused because of Russia's "blockage" so the parties could carry out talks in their capitals.

"We all have an interest in agreeing a deal," said the diplomat, adding that China has a "very important role to play" at this moment.

The EU's foreign policy chief Josep Borrell tweeted that the pause was "due to external factors," despite the fact that "a final text is essentially ready and on the table".

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

They had progressed most of the way toward their aim -- the revival of the 2015 Joint Comprehensive Plan of Action (JCPOA), which began unravelling when former US President Donald Trump withdrew from the deal in 2018.