US Secretary of State Mike Pompeo on Friday said that Iranian military commander Qasem Soleimani was planning imminent action that threatened American citizens when he was killed in a US strike.Soleimani, the commander of the Islamic Revolutionary Guard Corps' foreign operations arm, was killed by US forces in an air raid on Baghdad international airport."He was actively plotting in the region to take actions - a big action, as he described it -- that would have put dozens if not hundreds of American lives at risk," Pompeo told CNN."We know it was imminent," Pompeo said of Soleimani's plot, without going into detail about the nature of the planned operation.READ | Gold spikes as investors brace for Iran response to US strike"This was an intelligence-based assessment that drove our decision-making process," Pompeo added.Meanwhile, Iran's supreme leader Ayatollah Ali Khamenei vowed "severe revenge" after learning that Soleimani had been killed.Three days of mourning have been declared."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.ALSO READ | How Trump got into a showdown with Iran that could lead to war"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."At the same time, the US embassy in Baghdad urged American citizens in Iraq on Friday to "depart immediately" for fear of fallout from a US strike.