Sergei Lavrov, the Russian Foreign Minister, said it could amount to a violation of international law.
Sergei Ryabkov, Lavrov's deputy, said the EU's decision would force Russia to send the air defence system. "I can only say that we are going ahead with it," Ryabkov said. "We believe that such steps go a long way to restraining some hotheads from exploring scenarios in which this conflict could be given an international character with participation of outside forces."
As fears grew of an arms race in the region, Israel added to the growing sense of escalation by saying it would not stand by while the S-300 missiles were deployed.
"As far as we are concerned, that is a threat," Moshe Yaalon, the Israeli Defence Minister, said. "At this stage I can't say there is an escalation. The shipments have not been sent on their way yet. And I hope that they will not be sent. If, God forbid, they do reach Syria, we will know what to do."
The S-300 system has been described as a game-changer in the conflict and could help the Assad regime resist Western pressure. Russia said it would prove a stabilising factor in Syria.
Telegraph Group Ltd