Russia will pay the price for launching air strikes to prop up the Assad regime, the US defence secretary has warned.
Ash Carter said Russian President Vladimir Putin had "shattered the facade" that the country's military intervention in the war-torn state was to defeat so-called Islamic State.
The US and Britain have accused Russia of primarily targeting Western-backed rebel groups fighting Bashar al-Assad's government
“This will have consequences for Russia itself, which is rightly fearful of attacks," Carter said. "In coming days, the Russians will begin to suffer from casualties.”
ITV News International Affairs Editor Rageh Omaar said "Moscow isn't listening" and continues to believe an Assad-less Syria would only benefit violent extremist groups.
Speaking at a meeting of Nato officials, Carter accused Russia of "increasingly unprofessional behaviour", claiming it had fired cruise missiles at Syrian targets without giving any advanced warning.
In other developments in the crisis:
US officials claimed four Russian cruise missiles fired from the Caspian Sea had landed in Iran
Nato expressed concern at what it called Russia's "escalation" in Syria
The Syrian government said it had started a major offensive - with the help of Russian air support - to retake key areas of western Syria from "terrorists"
UK Defence Secretary Michael Fallon called on Russia to "stop propping up the Assad regime" and to use their influence "constructively"