"This strike has to be seen as a continuation of America's line to disregard the norms of international law," Ryabkov told journalists in Moscow, according to the TASS state news agency.
US shoots down Syria warplane after allies bombed
"What is this if not an act of aggression?" he said in Russia's first official reaction to the downing of the plane on Sunday.
"It is, if you like, help to those terrorists that the US is fighting against, declaring they are carrying out an anti-terrorism policy," Ryabkov said.
Violence in Syria
The US-led coalition said the Syrian plane had dropped bombs on its allies fighting against the Islamic State group in the war-torn country.
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