US-backed forces in Syria are racing to retake the country’s biggest dam from Isil amid warnings it is at serious risk of collapse.

Syrian Democratic Forces (SDF) on Monday inched closer, retaking a strategic military airbase just miles from the dam.

The Tabqa dam, which has been under Islamic of Iraq and the Levant control for more than two years, was put out of service by fighting in the area on Sunday, causing water levels to rise dangerously behind the structure and placing it at risk of collapse.

Isil issued a statement ordering residents of Tabqa and nearby Raqqa to evacuate, blaming US coalition air strikes.