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 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.
They now control a third of Afghanistan's regional cities and most of northern Afghanistan, in a crushing blow for government security forces.
Thousands of civilians have been displaced and fled to Kabul to try and escape the fighting.
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."
The US said it was sending military troops to the airport in Kabul to help evacuate a "significant" amount of embassy staff on special flights.
More than 1,000 civilians have been killed in Afghanistan in the past month, according to the UN.
Just this week thousands of people from northern provinces have become internally displaced, travelling to Kabul to seek safety. An estimated 72,000 children arrived in the capital in recent days and are mostly sleeping on the streets, according to Save the Children.