The Taliban have seized the cities of Ghazni and Herat in the same day, taking control of 11 provincial capitals in less than a week.
Taking strategically important Ghazni increases the likelihood the Taliban could take the capital Kabul.
There is also heavy fighting in the second largest city, Kandahar.
The insurgents have moved at speed, seizing new territories almost daily, as US and other foreign troops withdraw after 20 years of military operations.
They now control a third of Afghanistan’s regional cities, in a crushing blow for government security forces.
Taking Ghazni is a significant gain due to its location – the city is on the major Kabul-Kandahar motorway, linking militant strongholds in the south to Kabul.
A provincial council member in the city told the BBC that the Taliban had taken most of the centre, with only a police base on the outskirts controlled by the Afghan security forces.
Herat, in the west of Afghanistan, has been under siege for weeks and on Thursday a regional council member confirmed to the BBC that the city had also fallen to the Taliban, including the governor’s office. Video on social media shows the insurgents running through a central street firing their weapons, and the Taliban flag was seen flying over the police headquarters.
Meanwhile in Kandahar, once the Taliban’s stronghold, heavy fighting was reported and the militants claimed to have taken over its prison and released all of the inmates, although this has not been confirmed.
