Iran's armed forces chief of staff Major General Mohammad Bagheri warned of "severe revenge" for those behind the assassination Friday of a top nuclear scientist outside Tehran.

"Terrorist groups and the leaders and the perpetrators of this cowardly attempt should know that severe revenge awaits them," he said in a tweet reported by state news agency IRNA.

Bagheri called the death of Mohsen Fakhrizadeh "a bitter and heavy blow" and added "we assure (Iranians) that we will not rest until we have chased and punished" those involved.