Russia says it will go ahead with deliveries of S-300 anti-aircraft missiles to Syria, and that the arms will help deter foreign intervention.
Deputy Foreign Minister Sergei Ryabkov said the missiles were a "stabilising factor" that could dissuade "some hotheads" from entering the conflict.
Russia also criticised an EU decision not to renew an arms embargo on Syria.
Meanwhile, the BBC has heard evidence that 200 people were killed in a massacre in western Syria this month.
'We know what to do'
The S-300 is a highly capable surface-to-air missile system that, as well as targeting aircraft, also has the capacity to engage ballistic missiles.
It is broadly comparable to the US Patriot system which has been deployed by Nato to guard Turkish air space against attack, says BBC diplomatic correspondent Jonathan Marcus.
Mr Ryabkov said the contract for the missile systems had been signed several years ago.
"We consider these supplies a stabilising factor and believe such steps will deter some hotheads from considering scenarios that would turn the conflict international with the involvement of outside forces," he was quoted as telling journalists, in a coded reference to the use of Nato warplanes in Libya.
Mr Ryabkov added that the EU's decision not to renew its arms embargo on Syria would directly harm the prospects for a proposed peace conference next month.
