The Balkan nation had already invited Serbian nationals in mid-January to leave Iran and not to travel there, as the country's clerical authorities launched a bloody crackdown on a mass protest movement.

"Due to the deteriorating security situation, citizens of the Republic of Serbia are not recommended to travel to Iran in the coming period," the foreign ministry said in a statement on its website published overnight Friday to Saturday.

"All those who are in Iran are recommended to leave the country as soon as possible."

Iran said on Friday that it was hoping for a quick deal with the United States on Tehran's nuclear programme, long a source of discord between the two foes.

But Trump, after ordering a major naval build-up in the Middle East aimed at heaping pressure on Tehran, said on Friday that he was "considering" a limited military strike if the negotiations proved unfruitful.