"I apologise for my inability to continue serving and for all the shortcomings during my term in office," Zarif said in a message posted on his verified Instagram account.

Zarif thanked Iranians and "respected officials" for their support "in the last 67 months".

The resignation of Iran's top diplomat was confirmed by an informed source, however it can only take effect once President Hassan Rouhani accepts it.

Zarif has served as Rouhani's foreign minister since August 2013 but he has faced severe criticism from hardliners since the US withdrew from the nuclear deal in May 2018.