BEIRUT – Activist groups said Sunday that about 5,000 people were killed in Syria's civil war in August, the highest figure ever reported in more than 17 months of fighting as President Bashar Assad's regime unleashed crushing air power against the revolt for the first time.

The U.N. children's fund UNICEF put the death toll for last week alone at 1,600, the largest weekly figure for the entire uprising.

"The past month witnessed large massacres and the regime was conducting wide operations to try to crush the uprising," said Omar Idilbi, a Cairo-based activist with the Local Coordination Committees group. "Last month's acts of violence were unprecedented."