Russia has not yet delivered any S-300 air defence missile systems to the Syrian regime and may hold back from shipping the weapons this year or even indefinitely, Russian media reports said Friday.
Syrian President Bashar al-Assad appeared to imply on Thursday that Russia had already shipped part of the highly controversial consignment.
But the Vedomosti daily cited a Russian defence industry source as saying it was unclear if the weapons would be delivered to Syria at all this year while the Kommersant daily quoted its source as saying that delivery was only planned in the second quarter of 2014.
The Interfax news agency quoted an arms export source as saying that any deliveries of the S-300s, were they to take place at all, would be made "no earlier than autumn".
Assad had said in an interview with Al-Manar television broadcast Thursday: "All the agreements with Russia will be honoured and some already have been recently."
But both sources quoted by Kommersant and Vedomosti said that no delivery of the missiles had taken place yet. The contract was agreed in 2010 and according to Vedomosti is worth $1 billion.
Kommersant added that after delivery in 2014, a minimum of another six months would be needed for the training of personnel and tests before the systems were fully operational.
Interfax's source said that while a delivery in autumn was theoretically possible "much would depend on the situation in the region and the position of Western countries on solving the Syria conflict."
The source quoted by Vedomosti meanwhile said that while the Russian government is currently insisting in public that the contract will be fulfilled, this does not mean that the actual deliveries will ever take place.
According to Interfax, the deliveries could yet be put on hold indefinitely. Its source said this has been the case with Russian Iskander missiles that Syria had wanted several years ago but Moscow had refused to deliver.
The widely-admired missile systems are seen by analysts as having huge military importance for Assad in the conflict against rebels as the weapons could be used to ward off Western or Israeli air strikes against regime targets.
A report in Kommersant had also emphasised the risk of any third party like Israel seeking to destroy the S-300s once they were inside Syria, given that Russian specialists would be present on the ground to ensure they functioned properly.
"If just one single Russian citizen is hurt as a result, the political consequences are going to be very serious," an arms source told the paper. "No leadership with any common sense is going to take such a step," the source added.