Moscow promises to deliver sophisticated anti-aircraft missile system to Syria after arms embargo dropped.
Israel warned it "will know what to do" if Russia delivers a sophisticated anti-aircraft missile system to Syria which it promised to do after expressing anger over European Union moves to arm the country's rebels.
Moscow said it was being forced to send the S-300 system to the regime to prevent Western "hotheads" from intervening in Syria's civil war.
It came after Britain and France succeeded in persuading the EU not to renew an embargo on supplying weapons to opposition forces. William Hague, the Foreign Secretary, said the deal meant Britain could begin arming the rebels from now.
The move provoked anger in Russia, which has been implacably opposed to Western efforts to unseat President Bashar al-Assad.
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