The Taliban seized the cities of Ghazni and Herat on Thursday, in the most dramatic string of captures since launching their offensive.
Taking the 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.
The city of Qala-I-Naw also fell to the Taliban, who now control about a third of the country’s regional cities and most of northern Afghanistan. The group’s rapid advance has dealt a crushing blow to government security forces.
Thousands of civilians have been displaced and fled to Kabul to try and escape the fighting.
Taking Ghazni is a significant gain due to its location – it is 150km (93 miles) from Kabul, and 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, which has been under siege for weeks, is also a major win for the militants. It is Afghanistan’s third largest city, lies on significant ancient trade routes and is seen as the gateway to Iran.
On Thursday, video on social media showed the insurgents running through a central Herat street firing their weapons, and the Taliban flag was seen flying over the police headquarters. A regional council member confirmed to the BBC that the city had fallen to the militants.
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.
If Kandahar falls into militant hands, it will be the biggest loss yet for Afghan security forces.
“Life as we know it is getting harder day by day,” a woman in Kandahar told the BBC, adding “both sides are just using… us as pawns in their own power-hungry games”.
Prisoners were also freed in the southern city of Lashkar Gah, where there are reports that militants have taken over the police headquarters.
The US embassy in Kabul said it was hearing reports that the Taliban was executing Afghan troops who were surrendering, saying it was “deeply disturbing and could constitute war crimes.”