The Pope wrote in a letter, delivered to Vladimir Putin but addressed to all the G20 leaders meeting: "To the leaders present, to each and every one, I make a heartfelt appeal for them to help find ways to overcome the conflicting positions and to lay aside the futile pursuit of a military solution."
He added: "Rather, let there be a renewed commitment to seek, with courage and determination, a peaceful solution through dialogue and negotiation of the parties, unanimously supported by the international community.
"Moreover, all governments have the moral duty to do everything possible to ensure humanitarian assistance to those suffering because of the conflict, both within and beyond the country's borders."
The widespread killings in Syria had spiralled due to a lack of interest from the world, he said. "It is regrettable that, from the very beginning of the conflict in Syria, one-sided interests have prevailed and in fact hindered the search for a solution that would have avoided the senseless massacre now unfolding."