BRUSSELS >> Britain, France and Germany on Tuesday ratcheted up pressure on Iran to stop violating its landmark nuclear deal in a last-ditch effort to resolve their differences through talks while also starting a process that could bring back punishing U.N. sanctions on Tehran.

The three European Union countries are being pressed on one side by U.S. President Donald Trump to abandon the agreement like he did unilaterally in 2018, and on the other side from Iran to provide enough economic incentives for them to roll back their violations.

Now, the Europeans have reluctantly triggered the accord’s dispute mechanism to force Iran into discussions, starting the clock on a process that could result in the “snapback” of U.N. and EU sanctions on Iran.

The three nations specifically avoided threatening the sanctions while emphasizing hopes for a negotiated resolution. They held off their announcement until tensions between the U.S. and Iran had calmed down after the Jan. 3 killing of an Iranian general in an American drone strike so their intent would not be misinterpreted.

“Our goal is clear: We want to preserve the accord and come to a diplomatic solution within the agreement,” German Foreign Minister Heiko Maas said in a statement. “We will tackle this together with all partners in the agreement. We call on Iran to participate constructively in the negotiation process that is now beginning.”

Iran’s Foreign Ministry warned of a “serious and strong response” to the European move. But at the same time, ministry spokesman Abbas Mousavi held out an olive branch, saying his country was “fully ready to answer any good will and constructive effort” that preserves the nuclear deal, Iran’s official IRNA news agency reported.

The U.S. State Department said it fully supports the decision to initiate the dispute resolution mechanism. “We believe further diplomatic and economic pressure is warranted by nations,” it said in a statement.

“The civilized world must send a clear and unified message to the Iranian regime: Your campaign of terror, murder, mayhem will not be tolerated any longer,” Trump said, according to the statement.

The accord, which Iran signed with the U.S., Britain, Germany, France, China and Russia in 2015, has been unraveling since Trump pulled Washington out in 2018 and reinstated sanctions designed to cripple the Islamic Republic under what the U.S. called a “maximum pressure” campaign.

The Europeans felt compelled to act, despite objections from Russia and China, because every violation of the deal reduces the so-called “breakout time” Iran needs to produce a nuclear bomb, Britain’s Foreign Secretary Dominic Raab told Parliament.

“Each of these actions were individually serious,” Raab said. “Together, they now raise acute concerns about Iran’s nuclear ambitions.” Iran insists it is not seeking an atomic weapon.

At the time of the signing of the deal, known as the Joint Comprehensive Plan of Action or JCPOA, Iran’s “breakout” time was estimated to be as little as two months. With the safeguards in place, limiting Iran’s stockpiles of enriched uranium and heavy water, the number and types of centrifuges it can use to enrich uranium, and the purity that is allowed, that estimate grew to more than a year.

Trump said the deal should be renegotiated because it didn’t address Iran’s ballistic missile program or its involvement in regional conflicts, and reimposed U.S. sanctions that have left Iran’s economy reeling. To pressure the remaining signatories to provide enough economic incentives to offset the U.S. sanctions, Iran last year began violating its limitations in stages.

Throughout, it has announced the violations publicly and continued to allow inspectors from the International Atomic Energy Agency in to its facilities.

Following the U.S. drone strike that killed Revolutionary Guard Gen. Qassem Soleimani, Iran announced what it said was its fifth and final step in violating the deal, saying it would no longer abide by any limitation to its enrichment activities.

That left the Europeans “with no choice” but to invoke the dispute mechanism, Raab told Parliament.

“We do so with a view to bringing Iran back into full compliance,” he said, adding that they hope the move will “reinforce the diplomatic track, not to abandon it.”

In their letter to the EU’s foreign policy chief announcing their move, the three countries distanced themselves from the new U.S. sanctions.

“Our three countries are not joining a campaign to implement maximum pressure against Iran,” they said. “Our hope is to bring Iran back into full compliance with its commitments.”

At the same time, they rejected Tehran’s argument that it was justified in violating the deal because Washington broke the agreement first when it pulled out.

Invoking the dispute mechanism starts a 30-day period in which to resolve the problem, which can be extended and probably will be. If the problem persists, the matter could be brought before the U.N. Security Council and might result in the “snapback” of sanctions that had been lifted under the deal.

After receiving the letter, EU foreign policy chief Josep Borrell, who coordinates the agreement on behalf of the world powers, stressed that the pressure on Iran from Europe does not mean international sanctions will automatically be imposed.

Despite Iran’s recent violations, all remaining parties to the JCPOA have said it is worth preserving, saying it is the best way to curb Iran’s nuclear program. Diplomats note that even with its violations, Iran is still enriching uranium to a lower purity than it did before the deal, and IAEA inspectors continue to have access to its facilities.

“We see no reason for such a step,” the Russian Foreign Ministry said in a statement about the EU countries’ decision. It called the move an “ill-considered decision” that could lead to a new escalation and make a return to the original framework “unachievable.”

With the growing skepticism that the deal will be able to saved without U.S. involvement, British Prime Minister Boris Johnson suggested the possibility the agreement could be reworked somehow to address some of Trump’s concerns.

“Let’s work together to replace the JCPOA with the Trump deal,” he told the BBC.

Borrell refused to comment on that but again emphasized that the remaining signatories feel it is the best solution to limiting Iran’s nuclear ambitions.

“We have to preserve the nuclear deal and work to go back to full and effective implementation,” Borrell said in Strasbourg, France. He described the pact as a “significant achievement” and underlined that “there is no alternative to this agreement.”

Raab told Britain’s Parliament that “the government in Iran has a choice.”

“The regime can take the steps to de-escalate tensions and adhere to the basic rules of international law. Or sink deeper and deeper into political and economic isolation,” he said. “We urge Iran to work with us to save the deal.”