Germany, France, and the United Kingdom are expected to initiate the reimplementation of United Nations sanctions on Iran as soon as Thursday, three European officials told CNN on Wednesday.

This so-called “snapback” mechanism, built into the 2015 Iran nuclear deal, can be triggered as early as Thursday.

The 30-day process to reimpose sanctions that was agreed to as part of the 2015 deal leaves little time for diplomacy. European leaders are hoping that Tehran will use that time to get back to the negotiating table, open up its facilities for international inspectors and take concrete steps to come back into compliance with its nuclear obligations.

Iran, however, has threatened retaliation of “severe proportions” if the sanctions return, raising the specter of more instability in a region that was already on edge following the recent conflict.

Snapback Mechanism Expiring in October

Members of the Joint Comprehensive Plan of Action (JCPOA) — also known as the Iran nuclear deal — can trigger the snapback provision in the agreement to restore United Nations sanctions on Iran if the country is seen to be in violation of the deal. European members are said to be acting now so as to not miss this window to trigger the process as the provision is set to expire in October.

Iran has advanced its nuclear program well beyond the limits outlined in the JCPOA in the years since the United States, under former President Donald Trump, withdrew from the agreement. Tehran has maintained that its program is peaceful in nature but inspectors and analysts have said that its stockpile and enrichment is quickly approaching thresholds associated with weapons-grade material.

“It’s going back to the original JCPOA would be almost impossible,” Rafael Grossi, head of the International Atomic Energy Agency, told reporters Wednesday.

US Secretary of State Marco Rubio, who met with his European counterparts this week, described the snapback process as “a very powerful piece of leverage on the Iranian regime.”

Inspectors Return to Iran

Iranian parliament had passed legislation earlier in July to halt all cooperation with international inspectors. This legislation came on the heels of a 12-day conflict between Iran and Israel in which Israeli forces destroyed Iranian nuclear facilities in June. In retaliation, Iran fired missiles at Israeli cities and in the final days of the conflict, US forces struck three Iranian sites.

The International Atomic Energy Agency (IAEA) pulled its inspectors from the country in July citing wartime conditions. Satellite images from July showed damage to the entrances of Iran’s Isfahan Nuclear Technology Research Center.

Tehran has also threatened to withdraw from the Nuclear Non-Proliferation Treaty (NPT) if sanctions are reimposed. IAEA’s safeguards are based on the NPT, to which Iran is still a signatory.

Inspections, however, have since resumed. Grossi confirmed that inspectors were back at the Bushehr nuclear power plant on Wednesday.

“Today we are inspecting Bushehr,” he said to reporters in Washington. “We are continuing the conversation so that we can go to all places, including the facilities that have been attacked.”

Iranian Foreign Minister Abbas Araghchi said that the inspectors had been allowed back into Bushehr to monitor the replacement of fuel at the power plant. This decision was made by the Supreme National Security Council, he said.

Iran’s lawmakers had passed legislation to halt cooperation with the IAEA, but Araghchi denied that Tehran was seeking “new cooperation” with the watchdog.

Fallout From June Conflict

Tensions between Iran and Israel have been high since Israeli forces struck at Iran’s nuclear facilities in June, setting off a 12-day war.

Iran retaliated in kind, firing missiles at Israeli cities. In the last few days of the conflict, US forces also launched attacks, destroying three Iranian sites.

The IAEA pulled out its inspectors in July, citing an inability to work under wartime conditions. Satellite imagery in July also showed damaged entries to Iran’s Isfahan Nuclear Technology Research Center.

Tehran has accused the watchdog of giving Israel the pretext to attack by publicizing ways that the country was not in compliance with its safeguard rules.

Iran Divided Over IAEA Return

The decision to allow IAEA inspectors back into some of its facilities has prompted criticism from within Iran.

Parliamentary member Kamran Ghazanfari lambasted comments by Speaker Mohammad Bagher Ghalibaf, saying that the latter’s suggestion of limited IAEA inspections was an “explicit violation” of parliament’s laws suspending cooperation with the agency.

Iran’s parliament had passed the legislation in July after the conflict in June, couching it in terms of standing up to foreign aggression and the IAEA’s reporting that the agency was biased in its assessments.

Negotiations Resume

European negotiators met with Iranian representatives in Geneva on Tuesday in a final effort to head off sanctions. However, sources said that little progress had been made in the meetings.

US envoy Steve Witkoff had also been engaging with Iranian officials ahead of the conflict with the aim of a new nuclear deal. These efforts broke down when the fighting broke out.

Grossi told reporters Wednesday that he remained cautiously optimistic that the next month could be used to de-escalate tensions. “Don’t forget that there is still time, even if there is the triggering thing, there is a month, and many things could happen,” he said.

For now, Iran faces the possibility of new and renewed sanctions and divisions within its own government as the window for diplomacy begins to narrow. With the European members of the JCPOA expected to trigger the snapback mechanism as soon as Thursday, and that mechanism itself set to expire in October, the next month or so will show whether the renewed tensions with the West can be resolved by diplomacy or whether confrontation will define Iran’s next nuclear chapter.