BRUSSELS — Britain, France and Germany on Tuesday formally accused Iran of breaking the 2015 agreement that limited its nuclear program, taking the first step toward reimposing further United Nations sanctions.

The European countries triggered the dispute resolution mechanism under the 2015 deal, setting the clock running on what could be some 60 days of negotiations with Iran about coming back into full compliance. That process could end up with a “snapback” of United Nations sanctions on Iran that had been suspended under the deal, including an arms embargo.

The move, which had been expected for more than a week, was delayed when the United States killed a top Iranian commander, Maj. Gen. Qassim Suleimani, with repercussions that are still playing out in Iran and across the region.

The Europeans want to save the deal and persuade both Washington and Tehran to begin a new set of negotiations about missile development and Iran’s regional activities, a senior European official said.

But the three European countries, all signatories to the deal known as the Joint Comprehensive Plan of Action, or JCPOA, clearly felt that they had to respond to Iran’s movement away from compliance.

In a joint statement, the foreign ministers of Britain, France and Germany said that they had warned Iran on Dec. 6 that “unless it reversed course, we would have no choice but to take action within the framework of the JCPOA, including through the dispute resolution mechanism.”

But Iran “has chosen to further reduce compliance,” the statement said. “We have therefore been left with no choice,” but trigger the mechanism “in good faith, with the overarching objective of preserving the JCPOA and in the sincere hope of finding a way forward through constructive diplomatic dialogue.”

The three countries reiterated that they opposed Mr. Trump’s withdrawal from the deal and were not joining his campaign of “maximum pressure” to cripple Iran economically. “Our hope is to bring Iran back into full compliance,” they said.

There was no immediate reaction from Tehran.

The 2015 agreement restricted how many centrifuges Iran could use to enrich uranium — increasing the percentage of U-235, the rare isotope crucial to its use in nuclear fission — how highly it could enrich the metal, and how much uranium it could stockpile.

It has taken a series of steps beyond those limits, trying in vain to pressure the Europeans to make good on a commitment to ease the economic pain of American sanctions.

Earlier this month, in response to the Suleimani killing, Iran said it would no longer abide by any restrictions in its uranium enrichment, but without specifying what it would actually do, which was taken by the Europeans as a useful ambiguity.

Mr. Trump’s harsh sanctions include a prohibition on banking transactions with Iran — a very difficult prohibition to work around, given the global reach of American banks.

The Europeans have struggled to get a barter system working to circumvent the use of the dollar and American banking systems, adding to Iranian frustration as the United States sanctions take a toll on the country.

The Europeans, especially Prime Minister Boris Johnson of Britain, want to give a nod toward Mr. Trump while also trying to bring urgency to diplomatic efforts to get new talks underway.

In a BBC interview Tuesday morning, Mr. Johnson spoke flatteringly of Mr. Trump and said he wanted to avert further military confrontation between Iran and the United States. “Let’s dial this thing down,” he said.

“President Trump is a great deal-maker, by his own account,” Mr. Johnson said. “Let’s work together to replace the JCPOA and get the Trump deal instead,” he added, though it was not clear what such a deal would look like.

In a separate statement, Foreign Minister Heiko Maas of Germany said, “We could no longer leave unanswered the increasing Iranian violations of the nuclear agreement unanswered.”

“Our objective is clear: We want to preserve the agreement and reach a diplomatic solution within the agreement,” he said. “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.”

But there was some skepticism that the time was right for talks.

At the same time, Mr. Malley said, senior Iranian officials would not want to be seen talking with Trump administration officials responsible for the killing of General Suleimani, a very popular figure in Iran who was considered its second most important leader.

“The Europeans are trying not to offend Trump too much but also trying to keep the JCPOA alive,” he said. “But the U.S. now thinks it’s winning,” he said, “so the Europeans are trying now to avoid it all going off the rails.”

Mr. Malley said he did not agree with triggering the dispute mechanism now, given the newly complicated setting, but said that doing so was “is not fatal” to the deal. “The only way forward now is to use the time to try to get the U.S. and Iran to talk,” he added.

Last week Mr. Trump urged the European signatories to abandon the nuclear deal; as late as Sunday, the three European leaders pledged to salvage the deal and again urged Iran to return to full compliance.

Russia and China, also signatories to the nuclear deal, have supported the efforts to preserve it, including the purchasing of some Iranian oil.

In their joint statement, the three European foreign ministers urged all sides to negotiate and said: “Given recent events, it is all the more important that we do not add a nuclear proliferation crisis to the current escalation threatening the whole region.”