The Russian foreign ministry said in a statement that it was "deeply concerned" by news reports that Saudi Arabia was planning to buy Pakistani-made shoulder-launched surface-to-air missiles and anti-tank systems for armed Syrian rebels based in Jordan.
It said that the aim was to alter the balance of power in a planned spring offensive by rebels on the regime of President Bashar al-Assad.
"If this sensitive weapon falls into the hands of extremists and terrorists who have flooded Syria, there is a great probability that in the end it will be used far from the borders of this Middle Eastern country," the foreign ministry said.
Russia is widely seen as Assad's last remaining major ally in a conflict that has left an estimated 140,000 people dead since it began as a peaceful uprising in March 2011.