"I am apologising you for all the shortcomings...in the past years during my time as foreign minister ..."
Two sources familiar with the matter also told NBC News that it remained unclear whether Zarif's resignation would be accepted.
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.
Cliff Kupchan, chairman of the Eurasia Group and a longtime Iran watcher, described Zarif's resignation as a "bolt from the blue" from a diplomat who had served as "the face of Iran" for years. He then entered the Iranian diplomatic corps and was appointed foreign minister in 2013. "We'll see if it sticks", Secretary of State Mike Pompeo wrote on Twitter.More news: Johnson clinches 20th title with runaway Mexico win
"He has handed in his resignation, but that doesn't mean that this is the end of this story", said Trita Parsi, president of the National Iranian American Council in Washington.
He said that while it was still too early to determine what prompted Zarif's resignation, it didn't necessarily signal a major shift away from the nuclear deal. However, according to the semi-official ISNA News Agency, Zarif was not present at any of Assad's meetings with Iran's supreme leader Ayatollah Ali Khamanei and Rouhani. "It limits Iran's nuclear program in exchange for sanctions relief".
In 2018, hardliners launched an effort to impeach Zarif, but their efforts did not succeed. "But their attacks would not have become this potent and this problematic for him had it not been for the fact that the Iranian public, by and large, are very disappointed in the JCPOA not living up to its promises", Parsi said. A spokesman for the Iranian mission to the United Nations, Alireza Miryousefi, confirmed the resignation to Reuters.
The latest point of contention between Zarif and hardliners was the implementation of the Financial Action Task Force's requirements regarding money laundering in Iran. Even so, it is highly unusual for a senior Iranian official to resign on social media.