By Philip Pullella

VATICAN CITY (Reuters) - Pope Francis, in a letter to Russian President Vladimir Putin for the G20 summit, urged world leaders on Thursday 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 August 21 that killed hundreds of people.

The Vatican released the letter as the Holy See briefed ambassadors about the pope's initiatives, including an unprecedented international day of prayer for peace in Syria which he has called for this Saturday.

The U.S. and French envoys were among some 70 envoys at Thursday's meeting with Archbishop Dominique Mamberti, the Vatican's foreign minister.

"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," the pope said.

"To the (G20) 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 said the international community should instead press for "a peaceful solution through dialogue and negotiation".

G20 host 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. Assad's government denies it was responsible.

In the letter, Francis, the former Cardinal Jorge Mario Bergoglio of Argentina, asked the Russian president to pray for him.

Putin always attends major religious festivals of the Russian Orthodox Church and has deepened his ties with the Church in recent years as part of a conservative course he has taken as president to rally voters behind him following street protests by the opposition.

The Vatican has been pressing its message on Syria all week since the pope announced his peace initiative last Sunday.

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The Holy See's deputy justice minister said the Syrian conflict had "all the ingredients" to spark a global conflict and the leader of the worldwide Jesuit order said a military intervention by the United States or France would be "an abuse of power".

Francis 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.

He will lead a prayer service in St Peter's Square expected to draw tens of thousands of people. At about five hours, it will be one of the longest events ever at the Vatican.

(Additional reporting by Timothy Heritage in St Petersburg, editing by Barry Moody and Mark Trevelyan)