Syrian government forces carried out heavy air strikes on rebel positions in and around the northern city of Aleppo on Friday, aiming to repel a major Islamist-led offensive on areas controlled by President Bashar al-Assad.

Thursday’s attack, the most intense insurgent offensive in Aleppo in three years, aimed to build on recent advances against Assad by an array of groups fighting on separate fronts, including Islamic State and rebels backed by his regional foes.

Aleppo, 50 km (30 miles) south of the Turkish border, was Syria’s most populous city before the country’s descent into civil war. It has been partitioned into zones of government and insurgent control since 2012. […]