An Afghan official says the Taliban have attacked a third provincial capital in less than a week, with an overnight assault on the western city of Farah.

Provincial Governor Mohammad Shoaib Sabet said there was no immediate word of casualties early on Friday and that airstrikes have been carried out against the militant group.

The growing violence comes as a US envoy said he and the militant group have reached a deal "in principle" on ending America's longest war.

Read: US envoy shows Afghan president draft Taliban deal

Resident Shams Noorzai said the Taliban seized an army recruitment centre close to Farah's main police headquarters and set it on fire. He said all shops have closed and some people are trying to flee.

The Taliban over the weekend also launched deadly attacks on the capitals of Kunduz and Baghlan provinces.