(Adds more from letter, background, prayer day on Saturday)
VATICAN CITY, Sept 5 (Reuters) - Pope Francis, in a letter to Russian President Vladimir Putin for the G20 conference, on Thursday urged world leaders to "lay aside the futile pursuit of a military solution" in Syria.
The United States and France are considering military action against Damascus in response to a chemical attack on Aug. 21 that killed hundreds of people.
"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 in the letter.
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.
"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 U.S. President Barack Obama out of air strikes to punish President Bashar al-Assad for the chemical weapons attack, which the West blames on the Syrian government. (Reporting By Philip Pullella; editing by Barry Moody)