European officials have notified the United Nations that they are preparing to reinstate U.N. sanctions on Iran, setting in motion a mechanism known as “snapback” under the 2015 Iran nuclear deal, three European officials told CNN Wednesday. The officials, who asked not to be named, said the mechanism could be set in motion as soon as Thursday.

European officials are “waiting for the last hours,” one official said, leaving the possibility for diplomacy to play out over the next month. European leaders hope Tehran will use the time to negotiate with the U.S. and international community, open facilities to inspectors and take steps to meet its nuclear obligations.

Iran, however, has vowed to retaliate against any reimposition of sanctions. Such a move could further destabilize the Middle East, already reeling from recent conflict between Israel and Iran.

Snapback is a provision of the 2015 Joint Comprehensive Plan of Action, or JCPOA, which set terms to limit Iran’s nuclear development. In return, JCPOA members agreed to lift sanctions.

JCPOA members retain the ability to snap back sanctions through a mechanism that allows a member to request a return of U.N. sanctions on Iran. The snapback mechanism will expire in October.

Iran has since exceeded the JCPOA limits on nuclear development since the United States left the deal under former President Donald Trump. Iran has refused to meet with U.S. officials to negotiate a new deal.

Tehran has maintained its nuclear program is for peaceful energy purposes, but inspectors and analysts say Iran’s nuclear program has advanced to the point where it is within a hair of nuclear weapons-grade.

“It is clear, going back to the original JCPOA would be almost impossible,” Rafael Grossi, head of the International Atomic Energy Agency (IAEA), said Wednesday.

“I think it would be a very powerful piece of leverage on the Iranian regime,” U.S. Secretary of State Marco Rubio said after meeting with European counterparts this week to discuss snapback.

Iran’s parliament passed legislation last month that would halt cooperation with international inspectors. Despite that vote, IAEA teams have returned to Iran in recent days to monitor its nuclear development.

Grossi told reporters in Washington Wednesday that IAEA inspectors were at the Bushehr nuclear power plant on Wednesday. “Today we are inspecting Bushehr,” he said. “We are continuing the conversation so that we can go to all places, including the facilities that have been attacked.”

The IAEA’s inspection authority comes from the Nuclear Non-Proliferation Treaty (NPT), of which Iran is a signatory. It is reportedly one of several options Tehran could pursue if U.N. sanctions are reimposed. Withdrawing from the NPT would have major implications for Iran and the international community.

Iranian Foreign Minister Abbas Araghchi said that the inspectors were at Bushehr to monitor the replacement of nuclear fuel, a decision made by the Supreme National Security Council.

“It doesn’t mean that we have signed a new cooperation with them,” Araghchi said in an interview with state-run Islamic Republic of Iran Broadcasting.

Araghchi also downplayed the significance of allowing the inspectors to return to Bushehr. “We let them return for sure to check, but this is neither an achievement, nor a new chapter of cooperation,” he said.

Israel struck Iran’s nuclear facilities in June, kicking off 12 days of conflict. Israeli cities and interests were attacked by Iran. U.S. forces joined the conflict in the final days, bombing three sites in Iran.

IAEA inspectors withdrew its teams from Iran in July, saying that the war made monitoring impossible. In August, satellite images showed damage to entrances of Iran’s Isfahan Nuclear Technology Research Center.

Tehran accused the IAEA of effectively giving Israel an excuse to attack Iran by publicizing Iranian non-compliance with IAEA safeguards rules.

Tehran’s decision to allow IAEA inspectors back into facilities, even temporarily, has opened it up to criticism domestically. Parliamentary member Kamran Ghazanfari has written to Speaker Mohammad Bagher Ghalibaf, saying his public comments that implied limits on the inspections were an “explicit violation” of laws passed to suspend cooperation with IAEA.

Iran’s parliament passed a law after the June conflict that cited foreign aggression and what it called bias in IAEA reporting.