I sincerely apologize for the inability to continue the service and for all the shortcomings during the service.
"Thank you very much to the dear and honorable people of Iran over the past 67 months".
A Foreign Ministry spokesman, Abbas Mousavi, confirmed to the state-run IRNA news agency minutes later that Zarif had resigned but gave no reason for his departure.
Zarif, who has served as the Iran's top diplomat since August 2013, led the Iranian negotiation team that led to the 2015 agreement known as the Joint Comprehensive Plan of Action, an agreement reached with the United States, Germany, Russia, China, Britain, and, France.
On Sunday, Zarif criticized Iranian hard-liners in a speech in Tehran, saying: "We can not hide behind imperialism's plot and blame them for our own incapability".
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It is not clear at this time if Iranian President Hassan Rouhani will accept Zarif's resignation. That decision lies with Iran's supreme leader, Ayatollah Khamenei. Unconfirmed rumors circulated on social media shortly after the Instagram post went up suggested he had tried to resign on multiple occasions - and that Rouhani had finally accepted this time.
Within Iran, Zarif's standing in the country's political establishment took a hit when the United States withdrew a year ago from the nuclear deal, aimed at curbing Tehran's nuclear program, and the pact's achievements became less and less clear as Iran's economy nosedived.
Zarif replaced Ali Akbar Salehi, the current head of Atomic Organization of Iran. "It limits Iran's nuclear program in exchange for sanctions relief".
But the deal was later challenged by the administration of President Donald Trump, which pulled America out of the accord.
Kerry and Zarif's rapport extended beyond just the nuclear negotiations, however. Is Mr Zarif's resignation a tactic?
But if the foreign minister does reign, Toossi said, it would be a major shift in the domestic balance of power away from the "moderate, pragmatic, reformist faction that Zarif embodies, especially on foreign policy".