Pope Francis has condemned an "ignoble" attack on a Syrian bus convoy that killed more than 100 people as he made his Easter sermon.

The pontiff specifically addressed Saturday's deadly bomb blast in a call for peace during a sermon to thousands of followers in St Peter's square.

"Yesterday's was the latest ignoble attack on fleeing refugees," he said.

The blast hit buses carrying civilians trying to escape rebel territory in an evacuation deal between the warring sides.