An Afghan official says insurgents have overrun a strategic district in the southern province of Helmand, delivering a serious blow to government forces.

Mohammad Jan Rasulyar, deputy governor of Helmand, said on Monday that Taliban fighters have taken control of the Sangin district after days of fighting.

Only Afghan army facilities in the district have not been taken by the insurgents, he says. Fighting is still ongoing.

Rasulyar on Sunday took the unusual step of using Facebook to warn President Ashraf Ghani that Helmand was in danger of falling if central authorities failed to send help.

He says more than 90 security forces died fighting in the past month.

Helmand is an important Taliban base as it produces most of the world's opium, a crop that helps fund the insurgency.