Iran's rulers are also facing a string of other crises - energy and water shortages, a collapsing currency, military setbacks among regional allies and growing fears of an Israeli strike on its nuclear facilities - all intensified by Trump's tough stance.

China and Russia stood by Iran on Friday after the United States demanded nuclear talks with Tehran, with senior Chinese and Russian diplomats saying dialogue should only resume based on "mutual respect" and all sanctions ought to be lifted.

A letter U.S. President Donald Trump wrote to Iran's supreme leader in an attempt to jump-start talks over Tehran's rapidly advancing nuclear program has arrived in the Iranian capital.