The 10-year agreement struck in Vienna this week calls for a lifting of the sanctions that have crippled the Iranian economy in exchange for measures to ensure Iran does not build nuclear weapons.
But the historic accord also features a "snapback" mechanism to restore the sanctions almost immediately if Iran does not live up to its commitments.
According to the letter, dated Tuesday and obtained by AFP, the so-called P5+1 countries -- US Security Council permanent members Britain, China, France, Russia and the United States, plus Germany -- commit to extending this mechanism by five years after the nuclear accord expires.
The presence of a "snapback" mechanism was known before now, but the timescale it covers had been unclear.
In coming days the Security Council is expected to pass a resolution endorsing the Vienna agreement and opening the way for the gradual and conditional lifting of sanctions against Iran.