Pope Francis urged the Group of 20 leaders on Thursday to abandon the "futile pursuit" of a military solution in Syria as the Vatican laid out its case for a negotiated settlement that guarantees rights for all minorities, including Christians.

In a letter Thursday to the G-20 host, Russian President Vladimir Putin, Francis lamented that "one-sided interests" had prevailed in Syria, preventing a diplomatic end to the conflict and allowing the continued "senseless massacre" of innocents.

"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," Francis wrote as the G-20 meeting got under way in St. Petersburg.

Francis has ratcheted up his call for peace in Syria amid threatened U.S.-led military strikes following an Aug. 21 chemical weapons attack near Damascus.

He will host a peace vigil in St. Peter's Square on Saturday, a test of whether his immense popular appeal will translate into popular support for his peace message.

On Thursday, the Vatican summoned ambassadors accredited to the Holy See to outline its position on Syria, with Archbishop Dominique Mamberti, the Vatican's foreign minister, noting that the Aug. 21 attack had generated "horror and concern" from around the world.

"Confronted with similar acts one cannot remain silent, and the Holy See hopes that the competent institutions make clear what happened and that those responsible face justice," Mamberti told the 71 ambassadors gathered.

He didn't refer explicitly to the threat of military strikes to punish the Syrian regime for the attack. But he said the main priority was to stop the violence which he said risked involving other countries and creating "unforeseeable consequences in various parts of the world."

The Vatican, he said, called for a return to dialogue and for the country to not be split up along ethnic or religious lines.

Minorities, including Christians, must have their basic rights guaranteed, including their right to profess their religion, he said. And he called for the opposition to distance itself from extremists, isolate them "and openly and clearly oppose terrorism" — a reference to the Al Qaeda-affiliated rebels fighting against the government.

The Assad family's four-decade iron rule over Syria long has rested on support from the country's ethnic and religious minorities, including Christians, Shiite Muslims and Kurds. The Assad family and key regime figures are Alawites, followers of an offshoot of Shiite Islam, while most rebels and their supporters are Sunni Muslims.

As a result, the Catholic Church has toed a careful line on Syria, staying largely silent at the start of the civil war even after the regime's brutal crackdown on dissent. As the violence raged, the Vatican stepped up its call for dialogue. And amid the U.S. threat of military intervention, Vatican and church officials have warned that a world war could erupt, with Christians in the region bearing the brunt of the fallout.

On Wednesday, the head of Francis' Jesuit order, the Rev. Adolfo Nicolas, told a Catholic news agency that military action by the U.S. and France would be an "abuse of power."

"I cannot understand who gave the United States or France the right to act against a country in a way that will certainly increase the suffering of the citizens of that country, who, by the way, have already suffered beyond measure," he was quoted as saying in the interview, the text of which was released by a Vatican-affiliated spokesman.

This week, Syrian government troops have been battling Al Qaeda-linked rebels over Maaloula, a regime-held Christian village in western Syria. Some of its 2,000 residents still speak a version of Aramaic, the ancient language of biblical times believed to have been spoken by Jesus.