Taliban fighters could isolate Afghanistan's capital in 30 days and possibly take it over within 90, a US defence official cited US intelligence as saying, as the resurgent militants made more advances across the country.

The official, speaking to Reuters on condition of anonymity on Wednesday, said the new assessment of how long Kabul could stand was a result of the Taliban's rapid gains as US-led foreign forces leave.

"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.