The trio dubbed the E3 is due to initiate the sanctions as early as Thursday, four diplomats cited by Reuters said, but hopes Iran will make diplomatic concessions within the next 30 days.
The snapback mechanism, part of UN Security Council Resolution 2231 — which endorsed the 2015 nuclear deal called the Joint Comprehensive Plan of Action (JCPOA) — allows any party to the accord to file a complaint accusing Iran of non-compliance.
If no resolution is reached within 30 days, all previous UN sanctions would automatically resume and would include arms embargoes, cargo inspections and missile restrictions.
A round of nuclear talks involving Iran and France, Germany and the United Kingdom concluded in Geneva on Tuesday, with tangible progress announced.
Deputy foreign ministers from the four countries took part in the meetings which followed a previous round held in Istanbul in July.
The European powers have pressed Iran to resume talks with the United States and cooperation with International Atomic Energy Agency inspectors.
Both were put on hold amid a 12-day war launched by Israel on Iran which was capped off with US strikes on three key Iranian nuclear sites.
Iran denies seeking nuclear weapons but Israel and Western powers doubt its intentions.
Triggering the UN sanctions is likely to lead to complex, fast-paced diplomacy.
“There will be intense negotiation, diplomats say, over the form of that resolution. Among the questions are: will 2231 be extended and for how long, and will snapback clearly be allowed at a later date,” Wall Street Journal reporter Laurence Norman said on X Wednesday.
“The fundamental point is if there’s snapback tomorrow (Thursday), it’s not the end of diplomacy. It’s just going to proceed for a short period under new rules of the game,” he added.
Tehran on Wednesday warned of unspecified consequences if the countries triggered the sanctions mechanism and asserted they had no right to do so.
"In Geneva, we explained to the Europeans that, for legal reasons, they have no right to use the snapback mechanism, and that if such a thing happens, it will have consequences for them," Iran's foreign ministry spokesman Esmail Baghaei said.
"It was agreed that contacts between Iran and the European side would continue in the coming days," he added.