A top Russian diplomat has condemned the United States (US) for shooting down a Syrian Air Force fighter jet the previous day as an act of “aggression.”

The US military confirmed a US F-18 Super Hornet shot down Sunday a Syrian SU-22 after it dropped bombs near the US partner forces, known as the Syrian Democratic Forces, aligned with the Americans in the fight against the militant Islamic State (IS) group.

Russia has been a staunch supporter of Syria's beleaguered President Bashar Assad and has been providing an air cover for this offensive since 2015.

Russian Deputy Foreign Minister Sergei Ryabkov told Russian news agencies on Monday that the downing was akin to “helping the terrorists that the U.S. is fighting against.” Ryabkov asks: “What is this, if not an act of aggression? “