Germany, France and the United Kingdom are expected to trigger the reimposition of United Nations sanctions on Iran this week, three European officials told CNN on Wednesday.

The so-called “snapback” mechanism of the 2015 Iran nuclear deal could be set in motion as early as Thursday, beginning a 30-day process that European leaders hope will still allow for diplomacy.

Europe’s big three want Tehran to use that time to reengage in serious negotiations, open its facilities to international inspectors, and take steps to comply with its nuclear obligations.

But Iran has threatened a “severe response” if sanctions return, and officials fear the move could lead to more instability in the Middle East after recent fighting.

The snapback mechanism is set to expire in October

Iran’s violation of the 2015 nuclear deal with world powers, formally known as the Joint Comprehensive Plan of Action (JCPOA), would allow members to restore UN sanctions on Tehran under the snapback provision.

The authority for members to reinstate sanctions under the JCPOA will expire in October, if the 2015 deal does not get extended. That’s adding pressure on European countries to get the ball rolling this week.

Iran’s nuclear program has accelerated far beyond the deal’s limits since former President Donald Trump left the agreement. Tehran has insisted its program is peaceful, but its nuclear enrichment has reached capacities close to the weapons-grade level, according to inspectors and analysts.

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

U.S. Secretary of State Marco Rubio, who spoke with European counterparts this week to discuss the matter, called the snapback “a very powerful piece of leverage on the Iranian regime.”

Inspectors have returned to some facilities despite Iran’s threats

Iran’s parliament in July passed legislation ordering the government to no longer cooperate with international nuclear inspectors.

But IAEA teams have recently returned to facilities in Iran. Grossi on Wednesday confirmed inspectors were present at the Bushehr nuclear power plant.

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

IAEA inspections are based on the safeguards of the Nuclear Non-Proliferation Treaty (NPT), of which Iran is a signatory. The report said one of the options being considered by Tehran if sanctions are re-imposed is a withdrawal from the NPT.

Iranian Foreign Minister Abbas Araghchi said the IAEA inspectors were at Bushehr on Wednesday to monitor the ongoing fuel replacement at the plant, per a decision by Iran’s Supreme National Security Council. Araghchi denied the move was the start of “new cooperation.”

Iran also warned inspectors against publicizing any new violations as evidence for the snapback mechanism.

Tensions have been high since Israel’s strike on Iran’s nuclear facilities in June

Israel bombed Iran’s nuclear facilities in June, which sparked a 12-day conflict during which Iranian retaliatory missile attacks targeted Israeli cities.

In the conflict’s final days, U.S. forces were also involved, striking three Iranian sites in support of Israel. The IAEA pulled its inspectors from Iran in July, saying the state of war between Iran and Israel had made monitoring impossible.

Satellite imagery showed damage to doors and gates of entrances at Iran’s Isfahan Nuclear Technology Research Center. Iran later said the IAEA provided Israel with the pretext for its attack by publicizing the breaches.

Iran’s decision to allow inspectors back into some facilities has drawn criticism at home

Iranian parliamentary member Kamran Ghazanfari slammed Speaker Mohammad Bagher Ghalibaf over comments this week that Iran would permit limited inspections. Ghazanfari called Ghalibaf’s comments an “explicit violation” of the parliamentary law passed in July that suspended cooperation with the IAEA.

Iran’s parliament passed the legislation to suspend cooperation with inspectors in the weeks after the June conflict, which Iran said was meant to defend the country against foreign aggression and perceived IAEA bias in its reporting on Iran’s nuclear activities.