Saudi Arabia has hit back at Russian criticism of its reported plans to supply shoulder-launched missiles to Syrian rebels, saying it was Moscow's support for Damascus that was prolonging the conflict.

A spokesman told the official SPA news agency late Friday that the foreign ministry had been "astonished by Russian criticism of Saudi Arabia for its support of the Syrian people".

He said it was persistent Russian backing for Syrian President Bashar al-Assad, and its repeated use of its veto at the UN Security Council, that was blocking a peaceful solution.

"This support is the principal reason for the barbarity of the Syrian regime and for the conflict dragging on for three years without hope of a settlement or of an end anytime soon to one of the most serious humanitarian crises of our time."

On Tuesday, Russia warned Saudi Arabia against supplying Syrian rebels with shoulder-launched missile launchers, saying such a move would endanger security across the Middle East and beyond.

The Russian foreign ministry said it was "deeply concerned" by news reports that Saudi Arabia was planning to buy Pakistani-made surface-to-air and anti-tank systems for Syrian rebels based in Jordan.

"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 ministry said.

Long-existing tensions between Russia and Saudi Arabia have intensified further as a result of the Syria conflict, with Moscow standing by Assad but Riyadh offering open support for the rebels.

Russia and Iran are Assad's last major allies in a conflict that has left an estimated 140,000 people dead since it began as a peaceful uprising in March 2011.