Moscow has reportedly supplied Syria with three battalion sets of S-300 missile defense systems of eight launchers each for free amid its widening row with Israel, whose air force recently helped the downing of a Russian aircraft in the Arab country.

“On October 1, three battalion sets of S-300PM systems of eight launchers each were delivered to Syria,” the TASS news agency quoted a Russian military source as saying on Monday.

“These systems were previously deployed at one of the Russian aerospace forces’ regiments which now uses the S-400 Triumph systems. The S-300 systems underwent capital repairs at Russian defense enterprises, are in good condition and are capable of performing combat tasks,” it added.

In addition to the launchers, Russia had also provided Syria with more than 100 surface-to-air guided missiles for each S-300 battalion, the source noted.

The S-300 missile defense system – known as one of the most advanced in the world -- is capable of striking short- and medium-range ballistic missiles, cruise missiles as well as tactical and strategic aircraft.

Russia signed an agreement to sell the system to Syria back in 2010, but later scrapped the plan.

Last month, however, Moscow vowed to bolster Syria’s air defense capabilities by deploying the modern S-300 systems to the country.

The announcement came in the wake of the accidental downing of an Il-20 reconnaissance aircraft with 15 servicemen on board by Syria’s S-200 air defense systems, which were at the time responding to a wave of Israeli strikes on state institutions in Latakia Province.

Moscow held Israel responsible for the September 17 incident, saying the regime’s pilots had intentionally used the Russian plane as cover to conduct air raids, effectively putting it in the crosshairs of the Syrian air defenses.

Israeli Prime Minister Benjamin Netanyahu criticized Russia’s decision to supply the air defense system to Syria as “irresponsible,” saying Tel Aviv “will continue to do what it has to do to defend itself.”

Russian jets have been targeting positions held by terror outfits inside Syria at the Damascus government’s request since September 2015. The airstrikes have helped Syrian forces advance against the militants, who have been wreaking havoc in the Arab country since 2011.

On the contrary, Israel frequently attacks military targets in Syria in what is considered as an attempt to prop up militant groups that have been suffering heavy defeats against Syrian government forces.