Russia has confirmed it has sent an S-300 air defence missile system to its naval base in Syria's port of Tartus.
Defence ministry spokesman Igor Konashenkov said the purpose of the system was to guarantee the security of the base from the air.
The move comes amid growing tension with the West. On Monday, the US halted talks with Russia on trying to co-ordinate air strikes against jihadists.
A ceasefire brokered by Washington and Moscow collapsed last month.
"Let me remind you that the S-300 is a purely defensive system and poses no threat to anyone," Maj-Gen Konashenkov said.
"It is unclear why the deployment of the S-300 caused such alarm among our Western partners."
The spokesman said the system was similar to one earlier deployed at sea on the cruiser Moskva.