President Vladimir Putin on Wednesday revealed Russia had suspended deliveries of sophisticated S-300 missile systems to Syria even though some components had already been handed over.

Western governments have repeatedly condemned Russia for signing a contract to supply President Bashar al-Assad's regime with the S-300 air-defence systems which are among Russia's most prized military exports.

Russia has repeatedly insisted that the contract is being implemented but in an interview with Channel One state television Putin said deliveries had now been suspended, without making clear why.

"We have delivered separate components but the whole delivery has not been completed and for the moment we have suspended it," Putin said.

The announcement appears aimed at reassuring Russia's Western partners ahead of this week's G20 summit in Saint Petersburg, which is taking place against a background of huge diplomatic strains over the Syria conflict.

Putin had in June said the contract had not been "realised", saying Moscow did not want "to upset the balance in the region".

The West and Israel argued that delivery of the sophisticated weaponry by Moscow would hand Damascus the firepower to hit back at possible air attacks.

Putin acknowledged that the S-300 systems were not Russia's most modern weaponry but also could not resist opining that they are a "bit better than the (US) Patriots, for example".

The S-300 surface-to-air missile systems were first developed under the Soviet Union but new versions also appeared after the collapse of the USSR.

A military source told the state RIA Novosti news agency Wednesday that the components delivered by Moscow to Damascus were thus far not enough to mount a single complete S-300 missile system and could not be used as a weapon.

"Rosoboronexport (Russia's arms export agency) has not completely implemented the contract with the S-300 systems. Deliveries started but have not been completed. They cannot be used as a single defence unit," the source told the news agency.

The Vedomosti daily, citing sources in the arms industry, reported in August that several of the S-300s destined for Damascus had already been built.

Syria has paid a deposit of several hundred million dollars, the sources told the newspaper.

But Damascus is not expected to push for a quicker delivery of the systems or to demand its deposit back due to Assad's need for Russian support, it added.

According to Russian media reports, the contract with Damascus is for between three to six missile system units and worth $1 billion.

The suspension of deliveries to Damascus comes after Russia in 2010 cancelled a contract to sell Iran S-300 systems in a deal that was estimated to have been worth some $800 million.

Russia came under strong US and Israeli pressure not to go ahead with the sale of the weapons systems to Tehran which would have impeded any Western military action against the Iranian nuclear drive.

Tehran expressed fury but the director of the Russian weapons manufacturer Almaz-Antey which makes the S-300s said last month that the systems destined for Tehran had been dismantled, with some elements used elsewhere and other parts destroyed.

"The hardware that was destined for Iran no longer exists," said general director of Almaz-Antey, Vladislav Menshchikov, quoted by the Interfax news agency.