President Barack Obama sternly rebuked Syria on Friday about violence there in which sources said 22 pro-democracy protesters died.

"I strongly condemn the abhorrent violence committed against peaceful protesters by the Syrian government today and over the past few weeks. I also condemn any use of violence by protesters," the president said in a statement.

Sources said that 22 died in the southern city of Deraa during a day of pro-democracy protests across the country as demonstrations entered a fourth week of defiance of the government and against the rule of President Bashar al-Assad.

"I call upon the Syrian authorities to refrain from any further violence against peaceful protesters," Obama said.

"Furthermore, the arbitrary arrests, detention, and torture of prisoners that has been reported must end now, and the free flow of information must be permitted so that there can be independent verification of events on the ground," he said.