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 United Nations 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.