Germany, France and the United Kingdom will likely invoke the reimposition of United Nations sanctions against Iran this week, three European officials told CNN on Wednesday.

Referred to as a “snapback” mechanism, the move is part of the 2015 nuclear deal and could be set in motion as early as Thursday, the officials said.

The three-month process sets in motion a diplomatic window of 30 days, during which European leaders hope Tehran will reengage in serious negotiations, open its facilities to international inspectors and take steps toward compliance with its nuclear obligations.

Iran, which has threatened significant retaliation if sanctions are reimposed, has moved to expand its nuclear program far beyond the terms of the 2015 Iran nuclear deal since former President Donald Trump pulled the United States out of the agreement.

Despite Tehran’s insistence that the program is for peaceful purposes, inspectors and analysts say Iran is amassing the infrastructure and materials to produce weapons-grade nuclear material.

“Iran’s nuclear activities are racing beyond previous limits set under the 2015 deal,” Grossi, who met with U.S. officials and European leaders in Washington, said Wednesday.

Rubio said the move is “sending a message to Iran to the extent that they are willing to comply,” as part of a diplomatic effort.

The secretary of state, who also spoke with European counterparts Tuesday and Wednesday, called snapback “a very powerful piece of leverage on the Iranian regime.”

Inspectors Return

Even as Iran’s parliament passed legislation last week to end cooperation with international inspectors, IAEA teams have returned to facilities in recent days.

Grossi confirmed teams were in place at the Bushehr nuclear power plant Wednesday, a facility that is outside the parameters of the Iran nuclear deal but monitored by the U.N. agency.

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

IAEA safeguards are mandated by the Nuclear Non-Proliferation Treaty (NPT), of which Iran is a signatory. The NPT was reportedly one of several options on the table if sanctions were to be reimposed, with one official indicating Tehran was considering withdrawing from the treaty.

Iranian Foreign Minister Abbas Araghchi said the inspectors were only on site “to monitor and control the process of fuel replacement” at Bushehr, noting the decision to allow that activity was made by the Supreme National Security Council. He denied a new agreement for “new cooperation.”

Fallout From Recent Conflict

Iran and Israel both blame each other for starting a 12-day war in June that saw attacks by Iranian forces on Israel and then a series of Israeli strikes on Iranian nuclear facilities. On the final day of fighting, U.S. forces struck three sites in Iran.

The IAEA evacuated its inspectors in July, saying conditions on the ground were too unsafe to work or move around. The agency released satellite images last week that showed entrances to one of Iran’s Isfahan Nuclear Technology Research Centers had been damaged.

The IAEA is reportedly considering revealing the damage at the Isfahan site. Tehran has accused the IAEA of helping Israel justify its strikes by publicizing Iranian non-compliance with safeguard rules.

Divisions Inside Iran

Iran’s decision to allow inspectors back into some facilities has drawn the ire of domestic officials. Parliamentary member Kamran Ghazanfari called out Speaker Mohammad Bagher Ghalibaf over comments made earlier this week in support of limited inspections by the IAEA, calling them an “explicit violation” of legislation passed by the body suspending cooperation with the agency.

The legislation had been passed after the June conflict. At the time, the parliament said the decision was meant to protect the nation from “attacks and psychological warfare by foreigners” and to prevent what it described as the IAEA’s “bias against Iran.”

European negotiators met with Iranian counterparts in Geneva on Tuesday in a final-diplomacy effort to head off snapback but sources said little progress was made.

Ahead of the outbreak of the June conflict, U.S. envoy Steve Witkoff had been making overtures to Iranian diplomats about new nuclear agreements that could take in place of the 2015 deal, but the fighting ended those negotiations.

Grossi said he hopes the next month will see “de-escalation” of the situation.

“Iran is under pressure from the outside and from the inside, and one has to see how this develops,” Grossi said, also referring to the domestic political pressure Iran faces over the snapback mechanism. “Don’t forget that there is still time, even if there is the triggering thing, there is a month and many things could happen.”

The Associated Press contributed to this report.