Insurgents breached Syria’s largest city Friday and clashed with government forces for the first time since 2016, according to a war monitor and fighters, in a surprise attack that sent residents fleeing and added fresh uncertainty to a region reeling from multiple wars.

The fighting is a significant escalation since rebels launched a surprise offensive earlier this week, seizing towns and villages as they cut a path toward Syria’s second-largest city.

Jihadists and their Turkish-backed allies shelled Syria's second city Aleppo on Friday, in a major offensive against government troops that has sparked some of the deadliest fighting the country has seen in years.