Europe's big three of France, the UK and Germany appeared set to kick-start a UN process to reimpose sanctions on Iran on Thursday.
Meetings on the move were held two days after a round of diplomatic talks in Geneva reportedly failed to lead to a breakthrough.
The mechanism, known as snapback, would reinstate six suspended UN Security Council resolutions on Iran, which has threatened to withdraw from the nuclear non-proliferation treaty should it be triggered.
Inspectors from the UN's nuclear agency entered Iran this week for the first time since a 12-day war with Israel in June. But Iran said this did not amount to any "final agreement on co-operation" with the International Atomic Energy Agency.
With the three European countries - known as the E3 - set to announce a decision as soon as Thursday, officials said Iran could still provide commitments within 30 days over its nuclear programme that will convince the countries to defer concrete action.
"If the E3 proceed, it's not a process without off-ramps over the next 30 days, but it would hinge on breakthroughs that haven't been made in the weeks leading up to triggering," Naysan Rafati, senior Iran analyst at Crisis Group, told The National.
The talks in Geneva ended without a conclusive result but the Europeans were continuing to seek a diplomatic solution, a German Foreign Ministry representative said. "We remain open to dialogue with the Iranian side," they said.
Iran's deputy foreign minister, Kazem Gharibabadi, who attended talks in Geneva on Tuesday, urged the trio to "make the right choice, and give diplomacy time and space."
European officials have not publicly stated which day they would trigger the snapback but have repeatedly said they were ready to do so before the end of August. They will lose that ability in mid-October as a 2015 nuclear deal with Iran expires.
“European leaders are nearing a decision on starting the snapback process against Iran. It would take a diplomatic miracle to prevent that from happening," an Axios reporter quoted an unnamed senior EU diplomat as saying. "Triggering the snapback process would not mean the end of diplomacy. The E3 leaders remain open to engaging with Iran in the weeks until the UN sanctions will come into effect."
France, the UK and Germany, collectively known as the E3, believe Iran is in breach of the 2015 nuclear deal, with its uranium stockpiles representing more than 40 times the agreed limit despite Iranian insistence that its atomic programme is peaceful. The E3 also wants Tehran to re-engage in talks with the US and resume full co-operation with the IAEA.
The agency's inspectors left Iran after Israel launched its unprecedented attack on June 13, striking nuclear and military sites, as well as residential areas, killing more than 1,000 people. It was followed by further attacks by the US.
The E3 received no response from Iran after suggesting last month during talks in Istanbul that the nuclear deal be extended by six months to allow more time for negotiations.
"There is certainly a view among some in Iran that the impact of snapback is likely to be marginal – US sanctions are more economically impactful and Iran's allies at the Security Council could try to limit the effectiveness of restored UN restrictions," Mr Rafati said.
"That view is reinforced by a sense that even if it were to agree to the E3's proposals, the possibility of snapback would only be deferred. But the fact that Tehran has engaged the E3 in hopes of avoiding snapback also shows that the impact cannot be ignored: the sanctions are broad and binding on UN members and can still still have a psychological and practical impact."
Russia has meanwhile circulated a draft at the UN Security Council also calling for a six-month extension but on the condition that snapback cannot be triggered during that period. Russia, which takes the helm of the UNSC in October, is a close ally of Iran, whose sole civilian power plant in Bushehr is fuelled by uranium imported from Russia.
