BRUSSELS (NYTIMES) - Russian demands that a revised nuclear agreement with Iran shield it from sanctions imposed because of its war in Ukraine halted efforts to revive the deal on Friday (March 11), just as negotiators said they had all but finalised the agreement.

The breakdown in talks delays any prospect of a deal, and risks scuttling it entirely, allowing Iran to move closer to the ability to build a nuclear bomb.

More immediately, the lack of a deal also delays the resumption of Iran's ability to sell oil on the world market, which Western countries hoped would ease soaring energy prices.

For 11 months, negotiators have been working to restore the 2015 nuclear agreement, which placed important limits on Iran's nuclear programme and lifted punishing economic sanctions on Iran imposed by the United States.

US and European negotiators have been warning for weeks that the nuclear talks could not be prolonged much longer because of advances in Iran's nuclear work.

The chief French negotiator, Mr Philippe Errera, said in a Twitter message: "A good deal is on the table. There is a critical urgency to conclude and a real risk that, failing this, the agreement will fall apart."

His British counterpart, Ms Stephanie Al-Qaq, said in her own message that "external factors must be resolved in next few days or agreement likely to unravel", adding that there is a "fair and comprehensive deal on table - ready for conclusion".

But Russia, as a signatory to the 2015 agreement, has tried to use its final approval of the revived Iran deal as leverage to open a loophole in the sanctions levied against it since it invaded Ukraine last month.

European officials said that they were working to find ways around the Russian demands, if necessary, but that neither the United States nor the Europeans would be willing to negotiate a broad exemption for Russia from Ukraine sanctions to allow it to conduct normal business with Iran.

Experts disagree about whether Moscow's approval is legally required for the nuclear deal to be restored.

But China and Iran may not want to proceed without Moscow, and Russia is a member of the joint commission that supervises compliance.

Russia also has the responsibility under the agreement for taking control of Iran's excess enriched uranium and working with Teheran to convert its Fordow nuclear plant into a research facility.

In principle, China, Britain or France could take on those tasks instead.

The complications first arose on Saturday, when Russia's Foreign Minister, Mr Sergey Lavrov, said he wanted a written guarantee that sanctions "launched by the US will not in any way harm our right to free, fully fledged trade and economic and investment cooperation and military-technical cooperation with Iran".

The demand has tangled the Ukraine war with the Iran nuclear talks, two pressing issues that US officials insist are separate.

The sanctions imposed on Russia after the invasion, said Secretary of State Antony Blinken, "have nothing to do with the Iran nuclear deal".

He said they "just are not in any way linked together, so I think that's irrelevant".

Both US and Iranian officials are eager to renew the deal: Iran desperately needs the lifting of sanctions that have crippled its economy, while the United States wants to restore the original deal's limits on Iran's ability to enrich uranium.

The deal broke down in 2018 when the Trump administration withdrew from it.

The Trump administration then applied a raft of new sanctions against Iran, and Iran began increasing its enrichment of uranium.

Iran has always denied it intends to build a nuclear weapon, but it is now considered to be within just several weeks of creating enough highly enriched uranium to create a bomb, even though it would take many more months to weaponise it.

Russia, however, has incentives to stop the deal's revival.

"There are two reasons why a deal right now would not benefit Russia," said Mr Ali Vaez, who follows the negotiations closely for the International Crisis Group.

"A nuclear deal with Iran would lift the weight of a major security crisis for the West and give it more space to put pressure on Russia. It would also decrease oil prices, and Iran's oil would enter the market."

But Russia's demands are not entirely clear, and go well beyond the framework of the Iran deal.

If Russia wants reassurances that it can do all the transactions required under the deal, that can be managed, European officials said.

If the demands are much broader, they might kill the deal entirely if they cannot be worked around.

Nor is it clear how big a loophole in the sanctions would be created if Russia got some of what it wanted.

If it is just a matter of a trading and commercial relationship with Iran, it may not be very important compared with the impact of other sanctions.

Although the deal is not dead yet, Iran analyst Trita Parsi wrote in the journal Responsible Statecraft, "Moscow does have the ability to harm the United States by delaying the agreement at a crucial point of Washington's vulnerability to high oil prices. It may also have the ability to pull the plug on the agreement."

"It remains unclear, however," he added, "if the Russian objective is to delay the deal to undermine the West's efforts to pressure Russia over Ukraine or to completely scuttle the deal."

Iranian officials have made it clear they want the deal - known as the Joint Comprehensive Plan of Action, or JCPOA - to go ahead without Russian interference.

Foreign Minister Hossein Amirabdollahian told Iranian media on Monday that Iran "will not allow any external factor to impact the national interests for removal of the sanctions".

That vow was repeated Friday by his spokesperson, Mr Saeed Khatibzadeh, who insisted that "no external factor will affect our joint will to go forward for a collective agreement".

He expressed the hope that the pause could provide "momentum for resolving any remaining issue and a final return" to the deal.

But there were fears in Iran and among analysts that Iran had privately backed down and was willing to sacrifice a deal, and live with continued sanctions, to appease its Russian ally.

"Iran is caught in a dilemma," said Mr Sadegh Al Husseini, an Iranian economist and analyst.

"Russia is its neighbour and closest regional ally. Iran sees that it needs to live with Russia and create the balance of power between Russia and the US."

Likewise, Mr Vaez said, "While desperate for sanctions relief, Iran can't afford to defy one of its sole reliable allies and agree to restore the JCPOA without Russia."

The deal was nearly done on Friday when Mr Josep Borrell Fontelles, the foreign policy chief of the European Union, which chairs the talks in Vienna, announced what he described as a "pause" in the talks "due to external factors".

"A final text is essentially ready and on the table," he said in a Twitter message.

His main negotiator in Vienna, Mr Enrique Mora, told reporters, "We're almost there. We have a text in which almost everything is done. We are at the limit of negotiating footnotes."

Mr Mora said work would continue to get all sides back to conclude the deal, "the sooner, the better."