VIENNA — France, Britain and Germany have threatened to trigger the “ snapback mechanism ” that automatically reimposes all United Nations sanctions on Iran over its nuclear program, saying Iran has willfully departed from their 2015 nuclear deal that lifted the measures.

The U.N. sanctions that were in effect before the 2015 deal are wide-ranging, including a conventional arms embargo, restrictions on ballistic missile development, asset freezes, travel bans and a ban on producing nuclear-related technology.

The European countries, known as the E3, offered Iran a delay of the snapback during talks in July in exchange for three conditions for Iran: resuming negotiations with the United States over its nuclear program, allowing U.N. nuclear inspectors access to its nuclear sites, and acco

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