The S-300

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 Lavrov 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.

Russia's envoy to Nato, Aleksandr Grushko, said Moscow was acting "fully within the framework of international law" in delivering the arms.

"We are not doing anything that could change the situation in Syria," he said.

"The arms that we supply are defensive weapons."

There had previously been a reluctance to mention the sophisticated weapons system by name, and the BBC's Jim Muir in Beirut says the latest Russian statements could be seen as an escalation.

There had been reports that Moscow was holding back on delivering the arms, in exchange for an Israeli commitment not to carry out further air raids over Syria, our correspondent says.

Israeli Defence Minister Moshe Yaalon said the Russian missile systems had not yet left Russia.

"I hope they will not leave, and if, God forbid, they reach Syria, we will know what to do," he said.

Russia has repeatedly blocked efforts to put more pressure on Syrian President Bashar al-Assad.

Along with the US, it has been leading efforts to organise an international peace conference on Syria next month.

The Syrian opposition has not said whether to attend the conference, and was locked in talks in Istanbul, Turkey, as an unofficial deadline to decide on its attendance passed.

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