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.

The EU diplomat who has been chairing the talks, Enrique Mora, told reporters that delegations had got to the point of "negotiating footnotes".

He praised in particular the United States and Iran for their "very constructive, very positive approach", adding that he hoped to see the talks resume "very, very soon".

However, 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.

"The Ukraine conflict has now entered the Vienna talks in a very real way," Eric Brewer of the Nuclear Threat Initiative told AFP.

He said the "blanket guarantee" demanded by Moscow "has thrown a wrench into this process at the last minute that really threatens to upend talks and prevent the restoration of the JCPOA".

The United States on Friday put the ball in Iran and Russia's court after the EU announcement.

"We are confident that we can achieve mutual return to compliance... (if) those decisions are made in places like Tehran and Moscow," State Department spokesman Ned Price told reporters.

- Energy 'weapon' -

One EU source close to the talks said that Russia had at first made "reasonable" requests related to its civilian nuclear activities in Iran, but that they were then broadened "outside the scope of the JCPOA".

Another diplomat from one of the European parties to deal said that "if the Russian block is confirmed to be definitive, we will be obliged to look at other options," adding that Moscow could not be allowed to "take the deal hostage".

The head of the British delegation Stephanie al-Qaq tweeted that she was "deeply disappointed" at the pause in the talks.

The last-minute hitch must be resolved in the "next few days", she warned, or else the agreement was "likely to unravel".

After he withdrew from the JCPOA, Trump went on to reimpose sanctions on Iran, including on its vital oil sector.

That prompted Iran to start disregarding the curbs laid down in the deal on its nuclear activity.

The JCPOA aimed to ensure Iran would not be able to develop a nuclear weapon, which it has always denied seeking.

"Russia's gambit may be to delay the revival of the deal in order to avoid a flood of Iranian oil on the market" and the subsequent fall in prices, Clement Therme, Iran specialist at France's Paul Valery University told AFP.

"In keeping prices high, the Kremlin can use energy as a weapon against the West," he added.

- Russia rejects blame -

As for Iran itself, "the Islamic Republic isn't in a position to counter the Russian strategy," Therme said.

"Moscow is making use of Iran's weakness."

Tehran itself has blamed the US for "creating challenges" in the final stage of the talks.

Iranian foreign ministry spokesman Saeed Khatibzadeh said in a tweet Friday that "no external factor will affect our joint will to go forward for a collective agreement".

Russia's ambassador to the UN in Vienna, Mikhail Ulyanov, told reporters outside the talks venue that he rejected "attempts to put all the blame on the Russian Federation", insisting that other parties to the talks "need additional time".

A European source said it was now up to Iran and China to apply pressure to Moscow to make sure the deal was not scuppered.