This is the assessment by US intelligence agencies, news agency Reuters reported, quoting a defence official.
“But this is not a foregone conclusion,” the official added, saying that the Afghan security forces could reverse the momentum by putting up more resistance against the insurgent group.
The warning comes amid reports of Taliban forces taking control of eight provincial capitals.
The last two cities to fall are Farah, capital of the same-named province, and Pul-e-Khumri in Baghlan.
Six of the other provincial capitals to have fallen since Friday are in the country’s north, with the insurgents setting their sights on Mazar-i-Sharif, the region’s biggest city.
Another report said that Afghan President Ashraf Ghani has replaced the army chief as government forces lose control of more territories to advancing Taliban forces.
Meanwhile, Washington’s special envoy Zalmay Khalilzad was in Qatar to try and convince the Taliban to accept a ceasefire.
Envoys from hosts Qatar, Britain, China, Pakistan, Uzbekistan, the United Nations, and European Union were also due to discuss the situation in Afghanistan, a source said.
The Taliban have appeared largely indifferent to peace overtures, and seem intent on a military victory to crown a return to power after their ouster 20 years ago in the wake of the September 11 attacks.
The UN’s International Organization for Migration said Tuesday that more than 359,000 people have been displaced by fighting this year alone.
(With agency inputs)