Rebel forces in Syria have taken control of the "majority" of the country's second-largest city, Aleppo, according to the UK-based monitoring group Syrian Observatory for Human Rights (SOHR).

Russia launched air strikes in response across parts of Aleppo overnight into Saturday for the first time since 2016, the observers added.

SOHR said more than 300 people - including more than 20 civilians - have been killed since the offensive started on Wednesday.

The offensive is the largest against the Syrian government in years and the first time rebels fighting the forces of President Bashar al-Assad have reached Aleppo since being forced out by the army in 2016.