Russia slammed yesterday's downing of a Syrian fighter jet by the US-led alliance - the first such escalation since the start of the conflict in 2011 - which CENTCOM said was "in defense of partners from the Syrian Democratic Forces", with Russia's Deputy Foreign Minister Sergey Ryabkov saying on Monday the attack was an "act of aggression" and assistance for terrorists that the US is fighting against.

“What is it then, if not an act of aggression, an act directly in breach of international law,” Ryabkov told journalists in Moscow, quoted by Russia's RIA news agency.

“If you want, it’s actually help for the terrorists the US is fighting, declaring that they are conducting a counterterrorism policy,” the deputy foreign minister added.

Ryabkov said he believed the strike “should be first of all regarded as the continuation of the US agenda of neglecting the norms of international law. Regardless of who has power in Washington, people there are used to the fact that there are circumstances allowing them to arrogantly look down on – and in some situations, to openly ignore – the basics of international relations.”

As reported on Sunday, a Syrian SU-22 warplane was shot down by a US F/A-18E Super Hornet on Sunday while it was on a mission in the countryside around Raqqa. Damascus stated that the plane was carrying out routine operations against ISIS terrorists when it was downed.

A statement by the Syrian Army released on Syrian state television said the plane crashed and the pilot was missing. The army statement said it took place on Sunday afternoon near a village called Rasafah. The "flagrant attack was an attempt to undermine the efforts of the army as the only effective force capable with its allies ... in fighting terrorism across its territory," the Syrian army said.

"This comes at a time when the Syrian army and its allies were making clear advances in fighting the Daesh (Islamic State) terrorist group," it added.

The U.S. Central Command later issued a statement saying the Syrian plane was downed "in collective self-defense of Coalition-partnered forces," identified as fighters of the Syrian Democratic Forces (SDF) near Tabqah. Washington says that the Syrian warplane “dropped bombs near SDF fighters south of Tabqah” - which may or may not include members of various al-Qaeda spin off groups, and was shot down in accordance with “rules of engagement” of Coalition partnered forces.

Before it downed the plane, the US said it had "contacted its Russian counterparts by telephone via an established "de-confliction line" to de-escalate the situation and stop the firing."

Below is the full statement that CENTCOM issued justifying the escalation.