The death toll from Syrian government air strikes on a rebel-held town outside Damascus neared 100 on August 17, as the UN's humanitarian chief expressed horror and appealed for civilians to be protected.

The series of raids on August 16 on the town of Douma, in the rebel bastion of Eastern Ghouta, was one of the bloodiest regime attacks in Syria's four-year war.

The air strikes came almost exactly two years after devastating chemical attacks on the same region that much of the international community has blamed on President Bashar al-Assad's regime.

The head of the Syrian opposition National Coalition, Khaled Khoja, called the August 16 attacks a "massacre" and pledged they "will not go unpunished."

The London-based Syrian Observatory for Human Rights said 250 people were also injured.

It said the government carried out a total of 10 air strikes near a busy market and that "most of the dead are civilians."

Based on reporting by AP, Reuters, AFP, BBC, and Al Jazeera