"Martyrdom was the reward for his ceaseless efforts in all these years," Khamenei said on his Farsi-language Twitter account in reference to Soleimani, also declaring three days of mourning.

"With him gone, God willing, his work and his path will not be stopped, but severe revenge awaits the criminals who bloodied their foul hands with his blood and other martyrs' in last night's incident".

Iran's Revolutionary Guards confirmed Soleimani had been killed early Friday, saying he was "martyred" in an attack by US helicopters on Baghdad airport.

Soleimani was one of the most popular figures in Iran and seen as a deadly adversary by America and its allies.

Khamenei called Soleimani the "international face of the resistance" and said he was killed by "the most cruel of those on earth".

All people who back the resistance would be his "avengers", he added.

"The lack of our devoted and dear general is bitter, but continuing the fight and achieving final victory will embitter the murderers and criminals even more," he added.