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

The latest Islamist 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 kilometres 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.

Aleppo is of vital importance to Assad, and losing it would further entrench a de facto partition of Syria between western areas still governed from Damascus and the rest of the country run by a patchwork of militias.

Fighting between the insurgents and government forces in Aleppo started on Thursday and raged into the early hours of yesterday while Syrian air and army strikes on rebel emplacements were continuous, the Syrian Observatory for Human Rights said.

Aleppo is of vital importance to Assad – losing it would further entrench partition of Syria

A Syrian military source said the attack had been repulsed and heavy casualties had been inflicted on the insurgents. He added that the air force and artillery had been used to target the rebels, who he said had used heavy weapons in their attack.

Later, state TV showed scores of bodies of armed combatants, saying the army had killed at least 100 insurgents in the counter-offensive.

The Observatory’s Rami Abdulrahman said rebel forces had seized some buildings from government control on the northwestern city outskirts of Jamiyat al-Zahra. He later said insurgents were making headway in a part of western Aleppo to get closer to the heart of the city.

At least 35 insurgents were killed in that area, including a dozen Syrians and many others of central Asian origin, Abdulrahman said. The Syrian war has drawn foreign fighters from across the Muslim world, including jihadists from central Asia.

Air strikes were also reported near the town of Azaz in the north of Aleppo, just over the border from Turkey. An insurgent alliance including the al-Qaeda-linked Nusra Front and the hardline Islamist Ahrar al-Sham said they had set up a joint operations room to run the offensive to “liberate” Aleppo and later govern it according to Islamic sharia law.