Syria's civil war is unleashing a "staggering humanitarian crisis" as hundreds of thousands of refugees flee violence, an international aid agency said.

Opposition activists said an air strike on rebel-held territory southwest of Damascus killed 20 people, including women and children.

More than 60,000 people estimated to have been killed in the 21-month conflict.

Over 600,000 Syrians have fled abroad, many to neighbouring Turkey, Lebanon and Jordan, as violence has spread and international efforts to find a political solution have sagged.

Refugees interviewed by the International Rescue Committee (IRC) cited sexual violence as a major reason they fled the country, the New York-based organisation said in a 23-page report on the crisis.

Gang rapes often happened in front of family members and women had been kidnapped, raped, tortured and killed, it said.

"After decades of working in war and disaster zones, the IRC knows that women and girls suffer physical and sexual violence in every conflict. Syria is no exception," the group said.

Rebels and government forces have both been accused of human rights abuses during the conflict, which began with peaceful protests against President Bashar al-Assad in March 2011.

The unrest turned violent after government forces fired on demonstrators and has since become a full-scale civil war.

Fierce winter weather has worsened the plight of hundreds of thousands of refugees.

The IRC urged donors to step up planning and funding in the expectation that more Syrians will flee.

"Nearly two years into Syria's civil war, the region faces a staggering humanitarian disaster," the report said.