VATICAN CITY -- Pope Francis on Sunday condemned the use of chemical weapons, but he called for a negotiated settlement of the civil war in Syria, and announced he would lead a worldwide day of fasting and prayer for peace there on Sept. 7.

Francis abandoned the traditional religious theme of the weekly papal appearance to crowds in St. Peter's Square and instead spoke entirely, and with anguish, about Syria.

"My heart is deeply wounded by what is happening in Syria and anguished by the dramatic developments" on the horizon, Francis said, in an apparent reference to the U.S. and France considering a military strike to punish the Syrian regime for a chemical weapons attack.

Francis reiterated previous appeals for all sides in the civil war to put down their arms and "listen to the voice of their conscience and with courage take up the way of negotiations."

With tens of thousands of people in the square applauding his words, Francis delivered his strongest remarks yet to express his horror at the use of chemical weapons.

"With utmost firmness, I condemn the use of chemical weapons. I tell you that those terrible images from recent days are burned into my mind and heart," the pope said, in an apparent reference to photos and TV images of victims of chemical weapons in Syria.

"There is the judgment of God, and also the judgment of history, upon our actions," he said, "from which there is no escaping."

Usually soft-spoken, Francis raised his voice as he declared, "War brings on war! Violence brings on violence."

His admonishment against resorting to arms as a solution recalled the repeated emotional implorations a decade ago by the late Pope John Paul II in a vain attempt to persuade the U.S. administration then led by President George W. Bush not to invade Iraq.

The deteriorating drama of Syria inspired Francis to set aside Sept. 7 as a day of fasting and prayer for Syria.

Francis invited Catholics, other Christians, those of other faiths and non-believers who are "men of good will" to join him that evening in St. Peter's Square to invoke the "gift" of peace for Syria, the rest of the Middle East and worldwide where there is conflict.

"The world needs to see gestures of peace and hear words of hope and of peace," Francis said.

He said the prayer vigil in the square will last from 7 p.m. until midnight.