The United Nations top human rights official has called for international action to protect Syria's civilians, saying its "ruthless repression" of anti-government protesters could drive the country into full-blown civil war.

The death toll in the pro-democracy demonstrations that began in March against president Bashar al-Assad now exceeds 3,000, including at least 187 children, Navi Pillay said in a statement on Friday. At least 100 people have been killed in the past 10 days alone.

"The onus is on all members of the international community to take protective action in a collective manner before the continual ruthless repression and killings drive the country into a full-blown civil war," Ms Pillay, a former UN war crimes judge, said.

"As more members of the military refuse to attack civilians and change sides, the crisis is already showing worrying signs of descending into an armed struggle."