Taliban fighters have entered parts of Herat, Lashkar Gah and Kandahar.
They have made rapid gains in rural areas since it was announced almost all foreign troops would go by September.
But the fate of these key cities could be crucial amid fears of a humanitarian crisis and how long government forces will be able to hold out.
The fundamentalist Islamist militia is already thought to have captured up to half of all Afghanistan’s territory, including lucrative border crossings with Iran and Pakistan.
One MP in Kandahar told the BBC the city was at serious risk of falling to the Taliban, with tens of thousands of people already displaced and a humanitarian disaster looming.
Gul Ahmad Kamin said the situation was getting worse hour by hour, and the fighting within the city was the most severe in 20 years.
He said the Taliban now saw Kandahar as a major focal point, a city they want to make their temporary capital. If it fell, then five or six other provinces in the region would also be lost, Mr Kamin said.