BRUSSELS — Britain, France and Germany ratcheted up pressure on Iran Tuesday to cease its violations of a landmark nuclear deal, stressing that they want to resolve differences through talks while starting the clock on a process that could result in a so-called “snapback” of U.N. sanctions.

The three countries, which signed the international agreement in 2015 along with the United States, Russia and China, said in a letter to the European Union’s foreign policy chief that they had no choice but to trigger the deal’s “dispute mechanism,” given Iran’s ongoing transgressions.

The three said they rejected Tehran’s argument that Iran was justified in violating the deal because the United States broke the agreement by pulling out unilaterally in 2018.

“We have therefore been left with no choice, given Iran’s actions, but to register today our concerns that Iran is not meeting its commitments,” the countries said in a joint statement.

EU foreign policy chief Josep Borrell said the pressure on Iran from Europe does not mean international sanctions will automatically be slapped on the Islamic Republic.

The aim of the move by France, Germany and Britain is “to find solutions and return (Iran) to full compliance within the framework of this agreement, he said.

Hours later, Iran’s Foreign Ministry warned of a “serious and strong response” to the European move.

However, Foreign Ministry spokesman Abbas Mousavi said Iran is “fully ready to answer any good will and constructive effort” that preserves the nuclear deal. He was quoted Tuesday by the official IRNA news agency.

Lorne Cook and David Rising are Associated Press writers.