Iran's supreme leader Ayatollah Ali Khamenei vowed "severe revenge" after the United States killed the commander of the Islamic republic's Quds Force, General Qasem Soleimani, in Baghdad on Friday."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."Major escalationThe Pentagon said US President Donald Trump ordered Soleimani's "killing," after a pro-Iran mob this week laid siege to the US embassy.Trump tweeted out a picture of the US flag without any explanation, as the pre-dawn developments marked the most major escalation yet in a feared proxy war between Iran and the US on Iraqi soil.Early Friday, a volley of missiles hit Baghdad's international airport, striking a convoy belonging to the Hashed al-Shaabi, an Iraqi paramilitary force with close ties to Iran.Just a few hours later, the Revolutionary Guard Corps announced Soleimani "was martyred in an attack by America on Baghdad airport this morning."The Pentagon said Soleimani had been "actively developing plans to attack American diplomats and service members in Iraq and throughout the region."It said it took "decisive defensive action to protect US personnel abroad by killing Qasem Soleimani," but did not specify how.Iran's Foreign Minister Mohammad Javad Zarif slammed the US strike as "extremely dangerous and a foolish escalation."