Nato has urged Russia to end air strikes "on the Syrian opposition and civilians", days after Moscow began raids to support Syria's government.
Moscow says it is targeting Islamic State and other Islamist positions, but US-led allies and Turkey say government opponents are targeted.
Turkish F-16 fighter jets were scrambled after a Russian plane entered Turkey's air space on Saturday.
Nato said it meant Russia's actions "have reached a more dangerous level".
Russia said the incident was a "navigational error". The US said Turkey would have been within its rights to shoot the jets down.
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Syria conflict: Nato warns Russia on air strikes - BBC News