Higgins notes from the hundreds of videos he has analyzed that "rather than deploying all their most powerful weapons at once the air force appears to have escalated the air war."

He says "L-39s [a Czech-made jet trainer that can also fly combat missions] and OFAB 100-120 bombs [Russian-made fragmentation munitions] appeared after the battle for Aleppo began, and cluster munitions appeared after the Idlib highway was closed."

On Thursday, videos emerged from the city of Aleppo of an airstrike against a neighborhood in rebel hands. They showed the bodies of men, women and children, caked in white dust and blood, being pulled from piles of rubble by frantic rescuers. Above, the top floors of an apartment block have been sheared off, raining lumps of concrete on the street below. Opposition activists estimated at least 15 people were killed.

Other videos uploaded by opposition activists this week show the town of Maarat al Numan, captured by rebels in October, enduring a fourth week of air-raids. The town sits on the main highway linking Damascus with the north, an important artery that is no longer under government control.

Fawaz Gerges, Professor of Middle Eastern politics and international relations at the London School of Economics, tells CNN: "I think the opposition is chipping away at the government's position. I think the opposition strategy is a war of attrition to exhaust the government's forces."

After spending several days among rebel fighters, Damon says that "On the one hand there's a certain sense of anger and frustration with the international community because they feel they have been abandoned. At the same time they haven't lost. They are making major progress and there is a sense of determination and belief that they can do this on their own."