VATICAN CITY - Pope Francis has called for the world to unite in a day of fasting and prayer for Syria on Saturday and said "God and history" would judge anyone using chemical weapons.
"May the cry for peace enter the hearts of everyone," the pope told tens of thousands of pilgrims at his traditional weekly blessing in the Vatican on Sunday.
"I condemn with particular force the use of chemical weapons. I still have in my mind and heart the terrible images of the past days," he said.
"There is judgment from God and history on our actions that no one can escape."
The pope announced he would lead the prayers with a five-hour vigil in St Peter's Square on Saturday.
He urged the international community to make "every effort" to begin a process of dialogue in Syria.
The pope's call came as the United States, France and other countries were making plans for possible air strikes against Syrian President Bashar al-Assad's regime over its alleged use of chemical weapons.
The Vatican has repeatedly pushed for a negotiated solution to the conflict and has already criticised plans by global powers for an armed intervention.
It has been more cautious, however, than some Christian leaders in Syria and the region who have openly defended the current Syrian government seeing it as a guarantor of a multi-religious country.
Christian minorities are concerned about the rise of radical Islam in the region and point to the example of Iraq, where there has been an exodus of Christians faced with violence and discrimination.
"My heart is profoundly wounded in these days by what is happening in Syria and I am anxious about the prospect of dramatic developments," the pope said.
"War leads to more war, violence to more violence," he said, voicing hope that "a chain of commitment for peace will unite all women and men of good will."
The pope said he extended his invitation to push for peace in Syria to all Christians and faithful from other religions, as well as non-believers.
Saturday's prayers in St Peter's Square will begin at 1700 GMT and end at 2200 GMT, he said.
"With all my strength, I call on the warring parties to listen to the voice of their own consciences, not to close themselves in their own interests but to look on the other as a brother," he said.
The pope's comments were his first explicit reference to the alleged use of chemical weapons in Syria.
In comments immediately following the claims, the Vatican's permanent observer to the United Nations in Geneva had cast doubt on the accusations.
"What immediate interest would the government in Damascus have in causing such a tragedy?" Silvano Tomasi told Vatican radio.
"Who does this inhuman crime really benefit?" he said.