"There are so many conflicts in this world which cause me great suffering and worry, but in these days my heart is deeply wounded in particular by what is happening in Syria and anguished by the dramatic developments which are looming," he said.
He also spoke to the use of chemical weapons in Syria.
"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. There is a judgment of God and of history upon our actions which are inescapable," he said.
Pope Francis called for the day of prayer and fasting to take place Saturday, Vatican Radio reported.
"We will gather in prayer and in a spirit of penance, invoking God's great gift of peace upon the beloved nation of Syria and upon each situation of conflict and violence around the world," he said.