"Strategic momentum appears to be sort of with the Taliban," Milley said, adding that "there's a possibility of a complete Taliban takeover, or the possibility of any number of other scenarios. I don't think the end game is yet written."
While the militants have yet to capture any of the country's 34 provincial capitals, they are pressuring about half of them. Afghan security forces are consolidating their positions to protect those major urban centers including Kabul.
The Afghan provinces have been the scene of heavy battles between Taliban and the government security forces since the start of withdrawal of the U.S.-led forces from Afghanistan early May.
U.S. President Joe Biden has set a formal end to the U.S. military mission in Afghanistan for Aug. 31. Last week, the U.S. Central Command said that 95 percent of the withdrawal had been completed.
Over 2,400 U.S. troops have been killed in Afghanistan over the past two decades, with 20,000 wounded, according to the Pentagon. Estimates show that over 66,000 Afghan troops have been killed, and over 2.7 million people have had to leave their homes.
On Tuesday, the Government of Afghanistan announced that it had a war plan to respond to the rapid advance of the Taliban amid the departure of international troops and unsuccessful peace talks. This announcement happened during the Muslim holiday of Eid-ul-Adha, the same day that the Taliban fired rockets in some areas of the country's capital.