An airstrike by Syrian government forces has killed at least 21 people in a busy market in Syria’s hotly disputed second city Aleppo, according to activists.

Dozens more were reportedly wounded in the strikes, which come as pro-regime forces continue to battle for control of the city against rebel forces. It also comes amid a simultaneous regime offensive in and around the capital Damascus, including the strategically-crucial Qalamoun region. Clashes between rebel forces and fighters loyal to the government of Bashar al-Assad – foreign Shia fighters from Iraq, Lebanon and Iran – have been engaged in a bloody struggle for the mountainous region for several weeks.

Around 400 people have reportedly been killed in airstrikes in Aleppo since the Syrian air force in ratcheted up its offensive on rebel strongholds in and around the city two weeks ago.

Many of the aerial attack by government forces have involved the use of “barrel-bomb” – crudely-made but powerful explosive devices, consisting of large metal drums rigged with explosives, usually dropped from helicopters. Human rights groups have condemned the use of such weapons, pointing out that their sheer inaccuracy has resulted in the deaths of countless civilians. 

Leading Syrian opposition figures have threatened to boycott the upcoming “Geneva 2” peace conference unless the Aleppo air offensive stops.