"In October, three battalions of S-300PM systems and their ammunition load were delivered to Syria. Later, a regimental command post was brought to the country. All the hardware makes part of the S-300PM equipment set, except for the reconnaissance and air target detection radar of the regiment’s command post, which is part of the S-300PM-2," the source said.

MOSCOW, October 19. /TASS/. Russian specialists are re-equipping S-300PM air defense missile systems delivered to Syria into S-300PMU-2 ‘Favorit’ complexes for their local operation, a source in military and diplomatic circles told TASS on Friday.

"Technical specialists of Russian defense enterprises who arrived after that started work to re-equip S-300PM into S-300-PMU-2 systems," the source said.

This work largely relates to "replacing Russian codes and letter frequencies of the ground interrogator and the letter frequencies of the radar for Syrian versions," the source explained.

"The Russian specialists will complete their work in late October, after which they will return home," the source said.

The agreement to deliver S-300 surface-to-air missile systems to Syria was signed back in 2010 but was later put on hold. On September 24, 2018, after the loss of Russia’s Il-20 reconnaissance plane in Syria, Defense Minister Sergei Shoigu said that Russia would transfer S-300 systems to the Syrian armed forces to enhance their combat potential. On October 2, Shoigu told Putin that the S-300s had been deployed to Syria.