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 so that they may all lay down their weapons," the pope told thousands of pilgrims at the Vatican on Sunday.

The pope announced that the prayers would be led by a five-hour vigil in St Peter's Square, telling the international community it should make "every effort" for peace.

His call came the United States and France were making plans for possible air strikes against Syrian President Bashar al-Assad's regime over its alleged use of chemical weapons.

"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.

"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 from history on our actions that no one can escape."

"Weapons and violence do not lead to peace, war leads to more war," he said.