Germany, France, and the United Kingdom will likely reinstate United Nations sanctions on Iran by triggering a so-called “snapback” mechanism, three European officials told CNN on Wednesday. The mechanism, which was part of the 2015 Iran nuclear deal, could be triggered as soon as Thursday.

The process takes 30 days to complete, which leaves a small window for diplomacy. European leaders hope that Iran will use that time to reenter into “meaningful negotiations,” open its facilities to inspectors, and make moves to comply with its nuclear obligations.

Iran has, however, threatened to retaliate against the snapback with harsh measures of its own, setting the stage for further turmoil in a region still reeling from recent conflict.

Iran has been steadily building out its nuclear program since the United States withdrew from the JCPOA under former President Donald Trump, which began that country’s exit from the accord. Tehran has announced that the program is for peaceful purposes, but the program has expanded so much that it is “very close to” weapons-grade material, according to inspectors and other experts.

“This week is critical, but going back to the original JCPOA would be almost impossible,” Rafael Grossi, head of the International Atomic Energy Agency, told reporters on Wednesday.

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

Inspectors Return Amid Hostile Atmosphere

International Atomic Energy Agency inspectors have returned to Iran in recent weeks despite legislation passed by Iran’s parliament in July that ordered a cessation of all cooperation with inspectors.

Grossi confirmed that inspectors were at the Bushehr nuclear power plant on Wednesday.

“We are continuing the conversation so that we can go to all places, including the facilities that have been attacked, because as soon as possible, we need to see what has happened, where there are problems, and what solutions need to be discussed with them,” Grossi told reporters in Washington.

The IAEA has a so-called safeguards system that is under the purview of the Nuclear Non-Proliferation Treaty (NPT). Iran, which has not withdrawn from the NPT, is still obligated to allow international inspections.

One of Tehran’s stated options in the event of sanctions snapback is withdrawal from the NPT.

Iranian Foreign Minister Abbas Araghchi on Wednesday said the decision to allow inspectors to monitor the replacement of fuel at Bushehr was made by Iran’s Supreme National Security Council. But he said there was no decision for “new cooperation.”

Tensions have ratcheted between Iran and its regional adversaries since the start of the 12-day Israel-Iran War in June, when Israel bombed Iran’s nuclear facilities. Iranian proxy militias targeted Israeli cities, and in the war’s closing days, U.S. forces also struck three Iranian sites.

The IAEA inspectors left Iran in July, saying they were unable to perform any monitoring work during wartime conditions. Satellite images later revealed that the entrances of Iran’s Isfahan Nuclear Technology Research Center were damaged in the Israeli strikes.

Iran later accused the IAEA of essentially providing Israel a pretext for the strikes by publicizing violations of Iran’s safeguard rules.

Allowing inspectors back into Iran’s facilities has attracted domestic criticism from those opposed to the move. Parliamentary member Kamran Ghazanfari lambasted Speaker Mohammad Bagher Ghalibaf for comments last week that expressed limited support for inspections.

Ghazanfari said that Ghalibaf’s words and actions, in effect, “explicitly violated” laws that were passed by parliament in July that suspend cooperation with the IAEA.

Iran’s parliament had passed the legislation in the wake of the June conflict, which Iran characterized as a defense against foreign aggression and apparent bias in IAEA reporting.

Negotiations Hit Wall

European negotiators met with Iranian representatives in Geneva on Tuesday in an eleventh-hour effort to prevent a snapback, but the officials said little progress was made.

Before the June war, U.S. envoy Steve Witkoff had been working to set the groundwork for a new nuclear deal in the months after the Trump administration’s departure. That process came to a halt as fighting began.

“I think now there is a month ahead, and even if there is the triggering thing, there is a month, and many things could happen. This is where we are at this moment,” Grossi told reporters in Rome on Wednesday.

For now, Iran faces pressure from both the West and within its own government. With the snapback mechanism set to expire in October, the next month will likely decide if diplomacy can withstand the latest attacks—or if sanctions and confrontation become the defining characteristics of Iran’s next nuclear chapter.