Russian air force strike aircraft began to attack rebel camps inside Syria on 30 September, with unconfirmed reports claiming that Moscow asked Washington to clear the required airspace of any US aircraft.
Israeli sources describe the new air activity over Syria as a "recipe for trouble", with Russian aircraft having bombed targets near Homs and Hama – both areas over which US aircraft are flying on an almost daily basis. Moscow's intervention could result in encounters between Russian and US aircraft, they caution.
The Russian military build-up in Syria has increased in recent days, with additional fighter aircraft and anti-aircraft systems having been deployed.
Israel and Russia earlier this month agreed to a set of procedures that are aimed at avoiding any potential encounters between their air forces over Syria.
One of Israel's main points of concern is the possibility that Iran could take advantage of the Russian move into Syria by also bringing in its forces. "This is a red line for us," one Israeli source notes.