US officials say they have been warned by Israel over the impending sale of an advanced Russian missile system to Syria, fearing it could hamper efforts for international intervention in the war-torn country.

Israel suspects that Russia plans to sell Damascus six S-300 missile batteries, as well as 144 missiles, the Wall Street Journal reported Thursday.

The long-range ballistic system, which can down both fighter planes and intercept cruise missiles, would represent a significant upgrade for Syria’s already formidable air defenses.

According to the report, Israeli officials say the deal has been in the works since 2010, when the government of Syrian President Bashar Assad began making payments for the $900 million package.

S-300 batteries in a Moscow parade. (CC BY www.Kremlin.ru)

S-300 batteries in a Moscow parade. (CC BY www.Kremlin.ru)

There have been several unconfirmed reports of the sale of the missile system to Syria over the past year. According to the Wall Street Journal, Assad has sought the batteries for several years to beef up the country’s air defense array.

Brig. Gen. Asaf Agmon, the director of the Fisher Institute for Air and Space Strategic Studies, told The Times of Israel recently that Syria’s air defense system, based on an earlier Russian technology, was already among the most advanced in the world.

Western media reported that the Israeli airstrikes on Iran-Hezbollah weapons transfers in Syria over the weekend were carried out from Lebanon, with pilots using a technique called “lofting” — essentially lobbing the bombs by speeding toward the border and pulling up at the last moment — rather than challenge Damascus’s air defense.

Russia has continued to back Assad despite the country’s two-year civil war and calls from the West for him to step down. US officials have been lobbying Putin not to let the sale go through, the Wall Street Journal reported, warning that Syria’s acquisition of the system could complicate efforts for outside military intervention.

On Tuesday, Moscow and Washington said they would work toward convening talks between opposition and regime forces in a bid to end the conflict, which has seen over 70,000 Syrians killed and over a million more displaced.

The US also announced it would provide another $100 million in aid to the rebels.

Mitch Ginsburg contributed to this report.