The proposed deal between two of the leading power brokers in Syria's devastating civil war was set out by Saudi intelligence chief Prince Bandar bin Sultan at a meeting with Russian President Vladimir Putin in Moscow last week, they said.

Russia has supported Assad with arms and diplomatic cover throughout the war and any change in Moscow's stance would remove a major obstacle to action on Syria by the UN Security Council.

Syrian opposition sources close to Saudi Arabia said Prince Bandar offered to buy up to $15 billion of Russian weapons as well as ensuring that Gulf gas would not threaten Russia's position as a main gas supplier to Europe.

In return, Saudi Arabia wanted Moscow to ease its strong support of Assad and agree not to block any future Security Council Resolution on Syria, they said.