US Iran envoy Brian Hook is leaving his post and US Venezuela envoy Elliott Abrams will add Iran to his role "following a transition period" with Hook, US Secretary of State Mike Pompeo said on Thursday.
Hook's departure comes as the United Nations Security Council prepares to vote next week on a US bid to extend an international arms embargo on Iran. Some diplomats have said the measure lacks support.
Pompeo did not give a reason for Hook's decision to leave.
It was not immediately clear when Hook would finish, but the announcement comes at a crucial time for US diplomacy on Iran.
If the United States is unsuccessful in extending the arms embargo on Iran, it has threatened to trigger a return of all UN sanctions under a process - known as snapback - agreed in the 2015 nuclear deal between Tehran and world powers.
Some diplomats have suggested the United States will likely start the snapback process, which could take up to 30 days, by the end of August.
Abrams, a Republican foreign policy veteran, was named US special representative for Venezuela in January of last year and has led a hardline approach aimed at ousting Venezuelan President Nicolas Maduro.
US officials have said privately that President Donald Trump has been frustrated by the failure to remove Maduro, who retains the support of the Venezuelan military, as well from Russia, China, Cuba and Iran.
