Russia said Tuesday the EU's "illegitimate" decision to lift its embargo on arming Syrian rebels would harm peace efforts and insisted its own delivery of sophisticated missiles to Syria was a deterrence against foreign intervention.

"This in and of itself is a rather controversial decision because arms supplies to non-state entities are forbidden by international law," Foreign Minister Sergei Lavrov said in reference to the EU's decision to lift an embargo against arming the rebels fighting Syrian President Bashar al-Assad.

"This is an illegitimate decision in principle," Russian news agencies quoted Lavrov as saying one day after talks on the Syrian crisis in Paris.

His deputy Sergei Ryabkov added that the embargo "directly harms the prospects of convening an international conference."

Russia, which is a key backer of Assad, and the United States, which supports the rebels fighting him, are jointly pushing the warring parties to agree to take part in an international peace conference in Geneva aimed at ending the more than two-year conflict, which has claimed some 94,000 lives.

Separately, in some of the most explicit comments to date on supplies of S-300 missiles to Syria by Moscow, Ryabkov said the anti-aircraft missiles acted as a "stabilising factor" in the Middle East.

"We consider these supplies a stabilising factor," he said, adding that they could act as a deterrence against foreign intervention.

Ryabkov's comments sparked a quick response from Israel, which claimed that Russia had not yet delivered any missiles to the Syrian government but said it "will know what to do" if it does -- an apparent allusion to another air-strike on Syria.

Ryabkov said Russia had signed a contract with the Syrian regime several years ago to supply the missiles, without specifying whether any deliveries had been made.

He also stressed that the weapons were purely defensive, insisting that they were designed to protect the regime against outside forces rather than serving to crush the domestic opposition.

Ryabkov reiterated that Russia's arms deliveries to Assad's forces were legitimate because they were conducted under pre-war agreements that were reached with an internationally recognised government.

Lavrov for his part accused Western nations of frustrating efforts to hold the proposed Syrian peace conference.

"A whole range of activities that are being undertaken -- not without participation, not without support of our Western partners including the United States and France --- objectively, intentionally or unintentionally work to disrupt the idea of convening the conference," he said.

He also said Iran should join the talks despite reservations from some Western countries such as France.

"The issue of Iran is key for us," Lavrov was quoted as saying.

Ryabkov argued that the fractured opposition's failure to appoint any representatives for the proposed conference was the biggest existing stumbling block to peace.

"The fragmented nature of the groups fighting the government and the inability of our partners including the United States and the EU to ensure a sufficient level of representation by the opposition at the conference are the main stumbling block today," he said.

"The European Union is essentially adding fuel to the fire of the conflict and reducing the chances of a conference being conducted on Syria."

The proposed "Geneva 2" meeting is expected to take place some time next month after US Secretary of State John Kerry and Lavrov agreed in May to try to bring the warring sides together at a conference.

Ryabkov also accused the 27-nation EU of setting "double standards" by lifting the embargo against the opposition but not Syrian President Bashar al-Assad's troops.