Moving further away from the 2015 nuclear agreement, Iran said Thursday it had stopped honoring the deal’s limits on research and development, a potentially important breach of the accord.

The step was Iran’s third retaliatory suspension of compliance with a provision of the accord since President Trump renounced it last year and reimposed severe sanctions aimed at crippling Iran’s economy.

Iran’s foreign minister, Mohammad Javad Zarif, conveyed the country’s latest step in a letter to Federica Mogherini, the top foreign policy official of the European Union, who has been trying to save the nuclear deal from unraveling.

Iran’s official Islamic Republic News Agency quoted a Foreign Ministry spokesman, Abbas Mousavi, as saying Mr. Zarif’s letter informed Ms. Mogherini that Iran had ceased all its commitments “in the field of nuclear research and development as of today (Thursday).”