Iran's Supreme Leader Ayatollah Ali Khamenei on Wednesday rejected holding negotiations with the United States over a nuclear deal, as a letter was delivered from U.S. President Donald Trump calling for such talks.

The US president says he has proposed talks on a deal to prevent Iran acquiring nuclear weapons and to avert military action.

Anwar Gargash's visit to Iran as an interlocutor would mark the Emirates' foray into mediating between Washington and Tehran, a role Abu Dhabi is uniquely placed to play.