BRUSSELS — The talks on reviving the 2015 nuclear deal with Iran were paused on Friday because of recent Russian demands that sanctions imposed on it because of its war on Ukraine exclude its dealings with Iran.

European officials are concerned that the pause may turn out to be the death knell for efforts to bring the United States and Iran back into compliance with the deal, which puts important limits on Iran’s nuclear program while lifting punishing economic sanctions on Iran imposed by the United States.

In particular, a deal would bring Iranian oil back on the world market at a time when Western countries are trying to wean themselves off Russian oil and gas, hoping to reduce their dependence on Moscow while they punish it for the invasion of Ukraine. Those hopes have now been cast in limbo.

Josep Borrell Fontelles, the foreign policy chief of the European Union, which chairs the talks in Vienna, confirmed the pause in a Twitter message. He said that “a final text is essentially ready and on the table,” but “a pause in Vienna talks is needed, due to external factors.”

He said he and his team would continue to try “to overcome the current situation and to close the agreement.”

The “external factors” that Mr. Borrell alluded to came when Russia began its invasion of Ukraine in late February. While 11 months of negotiations in Vienna have produced the “essentially ready” text to which Mr. Borrell referred, the United States and Iran did not manage to get the deal done before it was complicated by the Russian invasion of Ukraine.

At its base — as was the case when it was first negotiated in 2015 — the agreement seeks to deny or at least slow Iran’s ability to build a nuclear bomb, to give the rest of the world confidence that should Tehran choose to “break out” and build a weapon, there would be time to stop it.

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 weaponize it.

Since the invasion of Ukraine, Western governments have punished Russia with far-reaching economic sanctions and efforts to reduce or even ban imports of its oil. Lifting sanctions on Iranian oil sales would expand supply and help keep prices down, something that has made striking a deal even more urgent.

But Russia, which is a signatory to the accord, has tried to use final approval of the revived Iran deal as leverage to soften the economic sanctions against it by ensuring that its cooperation with Iran would not be sanctioned.

American officials called the two issues separate, but they have created complications since Russia was supposed to take Iran’s excess uranium, to be diluted for nuclear fuel, and pay Iran for it.

Russia’s foreign minister, Sergey V. Lavrov, on Saturday said he wanted a written guarantee that sanctions ‘‘launched by the U.S. will not in any way harm our right to free, fully fledged trade and economic and investment cooperation and military-technical cooperation with Iran.”

Following a call between Mr. Lavrov and his Iranian counterpart, Hossein Amir-Abdollahian, Russia said a restored nuclear deal must “ensure that all its participants have equal rights” to develop “cooperation in all areas” with Iran.

The United States quickly dismissed Russia’s demands.

The sanctions imposed after the invasion, said Secretary of State Antony J. 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.”

On Tuesday, Britain, France and Germany urged the completion of the deal. “The window of opportunity is closing,” they said in a statement. “We call on all sides to make the decisions necessary to close this deal now, and on Russia not to add extraneous conditions to its conclusion.’’

But Russia’s intentions are not yet clear. If Moscow wants guarantees limited to its obligations under the nuclear deal, that can be managed, officials say. If the Russian demand is broader, and includes exemptions from Western financial and trade sanctions, the deal could die.

Iran, which wants the deal done and the removal of the tough economic sanctions the United States has imposed on it, has criticized the new Russian demands. Mr. Amir-Abdollahian told Iranian media on Monday that Iran “will not allow any external factor to impact the national interests for removal of the sanctions.”

The spokesman for Iran’s foreign ministry, Saeed Khatibzadeh, insisted in his own Twitter message on Friday that “no external factor will affect our joint will to go forward for a collective agreement” and expressed the hope that the pause could provide “momentum for resolving any remaining issue and a final return” to the deal.

While Russia is a member of the pact — the United States withdrew in May 2018, and these negotiations have been to get Washington and Tehran back into compliance with the accord — Moscow’s approval may not be legally necessary. But China and Iran may not want to proceed without it, and Russia remains a member of the commission that oversees compliance.

The new deal envisions Russia taking Iran’s large stock of highly enriched uranium in excess of the deal’s limits, and it is not clear, officials say, whether any other country is ready to do the same.