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Germany, France and the United Kingdom will use the mechanism to trigger the reimposition of UN sanctions against Iran, three European officials told CNN Wednesday. The so-called “snapback” provision could be activated as early as Thursday as part of the 2015 Iran nuclear deal.

The process takes 30 days to complete, meaning there is only a short window for diplomacy. European leaders hope Iran will use that time to reengage in meaningful talks, open its facilities for inspection and take steps to show compliance with its nuclear obligations.

Iran, however, has threatened dire consequences if sanctions return, raising the specter of further instability in a region still reeling from a recent conflict.

European countries are using the snapback provision, part of the 2015 Joint Comprehensive Plan of Action (JCPOA), which allows members to reimpose UN sanctions on Iran if it is found to be in violation of the terms. The authority to trigger the snapback expires in October.

Iran has been moving quickly to expand its nuclear program far past the limits agreed to in the JCPOA after former President Donald Trump withdrew the US from the deal. Tehran claims its program is peaceful, but its growing capabilities are nearing weapons-grade levels, according to inspectors and experts.

“It’s very clear that going back to the original JCPOA would be almost impossible,” Rafael Grossi, director general of the International Atomic Energy Agency (IAEA), said Wednesday.

U.S. Secretary of State Marco Rubio, who held a coordination call with European counterparts Wednesday, called the snapback mechanism “a very powerful piece of leverage on the Iranian regime.”

Iran’s parliament had previously passed legislation that required the government to stop cooperating with international inspectors, but IAEA teams have been back on the ground recently. Grossi confirmed Wednesday that inspectors were at the Bushehr nuclear power plant.

“Today we are inspecting Bushehr. We are continuing the conversation so that we can go to all places, including the facilities that have been attacked,” Grossi said during a press conference in Washington on Wednesday.

The IAEA’s inspections are based on safeguards agreement established by the Nuclear Non-Proliferation Treaty (NPT), of which Iran is a signatory. One of the options Tehran is reportedly weighing should sanctions be reimposed is withdrawing from the NPT.

Iranian foreign minister Abbas Araghchi said the inspectors were there to monitor fuel replacement at the Bushehr plant following a decision by Iran’s Supreme National Security Council. He denied any larger agreement for “new cooperation.”

Iran’s nuclear facilities were the target of Israeli strikes in June that led to a 12-day conflict. Iran launched retaliatory attacks on Israeli cities, and US forces joined on the final days, hitting three Iranian sites.

The IAEA pulled its inspectors out in July, saying that the war made it impossible to do their job. Satellite images later showed damaged doors and protective covers at the Isfahan Nuclear Technology Research Center, one of the sites Iran has accused the IAEA of giving Israel intelligence to justify its strikes. Tehran has said the agency was complicit in the strikes by publicizing its own non-compliance with safeguards regulations.

Iran’s decision to let IAEA inspectors back into some of its facilities has been criticized by some in the country. Parliamentary member Kamran Ghazanfari rebuked comments by parliamentary speaker Mohammad Bagher Ghalibaf who said limited inspections were acceptable, calling his remarks an “explicit violation” of the legislation that suspended cooperation with the IAEA.

Iran’s parliament passed the legislation after the conflict in June, but framed it as a response to foreign aggression and what it said was bias by the IAEA in its reporting.

European negotiators met with Iranian representatives in Geneva on Tuesday in a last-minute effort to avert sanctions, but sources told CNN little progress was made.

U.S. envoy Steve Witkoff had been working ahead of the recent conflict in an effort to work out a new nuclear deal. But those talks came to a halt with the outbreak of violence.

Grossi expressed some hope that the coming month could lead to a de-escalation, though. “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 is under pressure from Western leaders and some of its own officials. With the snapback mechanism’s authority set to expire soon, the coming weeks will likely show whether diplomacy can hold – or if sanctions and conflict will mark the next stage of Iran’s nuclear program.