Iranian Foreign Minister Mohammad Javad Zarif stepped down on Monday, announcing his resignation on Instagram.
"Many thanks for the generosity of the dear and brave people of Iran and its authorities over the past 67 months. I sincerely apologise for the inability to continue serving and for all the shortcomings during my service. Be happy and worthy," he wrote, without giving a specific reason for his decision.
Zarif led the signing of the nuclear deal between Iran and six world powers in 2015, under which Iran agreed to curbs on its nuclear programme in return for the lifting of international financial sanctions..
But he was criticised by anti-Western hardliners in Iran after the US pulled out of the agreement last May and reimposed sanctions on Iran.
It was not clear on Monday whether President Hassan Rouhani would accept Zarif's resignation, although the Iranian news agency Tasnim confirmed the news, citing "some sources". A spokesman for the Iranian mission to the United Nations, Alireza Miryousefi, confirmed the resignation to Reuters.