The Israeli objections reflect a rift between Russia and the West over arms supplies to both sides in the complex multinational conflict which overshadowed the first day of the G8 summit in Northern Ireland yesterday.
The meeting of the leaders of the world's major industrialised powers is being attended by President Vladimir Putin whose government has remained a military ally of Assad and is opposed to proposals for a "no fly zone".
It is unclear whether the deliveries of the Russian S-300 weapons system have begun, and even why the Kremlin continues its unwavering military support for Assad.
"The Russians are like a book with the pages glued together," said the official.
But he said that the ostensibly "defensive" system - which the Russians intended as a deterrent to guard against Western intervention - could also be turned into an "offensive" threat against Israel given the missiles' range of 200km.
The Obama Administration announced last week that it would arm the Syrian rebels, citing Assad's limited use of toxic gas against the Syrian people in the conflict which has caused an estimated 94,000 deaths on both sides. But the Israeli press expressed scepticism that the move - which came as Assad appeared to be regaining the upper hand with the help of Hizbollah fighters from Lebanon - was solely prompted by Syria breaching President Barack Obama's "red line" linked to the deployment of chemical weapons.
Israel insists that it is not taking sides in the Syrian civil war raging on its border and officials have flatly denied reports that it would approve of Assad remaining in power.
The Israeli policy establishment is torn by a debate along the lines of "better the devil you know than the devil you don't know".
The Israeli Government says that Israel will act to protect its vital strategic interests and to defend the Israeli people. Last month the Israeli military struck military targets inside Syria three times in three days, while the Syrian conflict spilled over this month into the Israeli-occupied Golan Heights.
But Israel has not spoken publicly about the conflict on its border, and "not given advice to people", said another Israeli official.
The Israelis remain concerned about the possible transfer of weaponry to Islamic radicals who could in turn threaten Israel.
Asked whether the Israelis had criticised the US and Europeans for lifting the arms embargo against the Syrian rebels, one official said: "Are we enthusiastic about other arms being introduced into the theatre? No."
Another Israeli official said: "We haven't criticised them. But in private meetings we have urged them to be cautious."
