The United States and France are considering military action against Damascus based on conflicting reports of an alleged chemical attack on August 21 near capital Damascus.

"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," the pope said a letter to Russian President Vladimir Putin  for the G20 conference.

Francis said "one-sided interests" had prevailed and hindered the search for a solution to the Syrian conflict that "would have avoided the senseless massacre now unfolding."

The Middle East was "bitterly tested by strife" and needed peace, he added.

"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," Francis said in the letter.

He has called on the world's 1.2 billion Roman Catholics to join him on Saturday in a day of prayer and fasting to end the Syrian conflict.

He has invited members of all faiths to join him around the world in whatever way they see fit.

Putin is trying to talk US President Barack Obama out of air strikes on Syria which Moscow has warned would have catastrophic consequences.