Syrian rebels led by Islamist groups have begun a massive assault that aims to take full control of the northern city of Aleppo, the BBC reports.

Rebels fired hundreds of rockets and missiles into government-held areas in a multi-district attack, the Syrian Observatory for Human Rights said.

It said the Syrian military had returned fire and launched air raids.

Analysts say the fall of Syria’s second city would be a major setback for President Bashar al-Assad.

The UK-based Observatory said that four civilians had been killed and more than 70 wounded in the rebel offensive, while five rebel fighters were killed in clashes with government forces in western Aleppo.