Under Mr Massoud, the Panjshir fighters earned a legendary reputation for resistance, defending their mountain homes first from the Soviet military for a decade, then throughout a civil war, then the last Taliban regime from 1996-2001.
“With this victory, our country is completely taken out of the quagmire of war,” the Taliban’s chief spokesman Zabihullah Mujahid said in a statement.
Massoud’s portrait destroyed
Soon after, in a photograph released by Taliban official Bilal Karimi, the same picture of Mr Massoud is seen with his face ripped out. Taliban gunmen then stand posing in front of the ragged portrait.
The National Resistance Front (NRF) – made up of anti-Taliban militia and former Afghan security forces – have admitted to suffering heavy losses, and have called for a ceasefire.
On Sunday, the NRF said spokesman Fahim Dashty – a well-known Afghan journalist – and a top commander, General Abdul Wudod Zara, had been killed.
But they also said their fighters were still present in “strategic positions” across the valley, and that they were continuing the struggle.
The Taliban completed a stunning two-week offensive across Afghanistan on 15 August, taking the capital, Kabul, without a fight.
Remnants of the Afghan army then retreated to the Panjshir Valley, north of Kabul, to create the NRF.
The 115-kilometre-long valley surrounded by jagged snow-capped peaks offers defenders a natural military advantage, allowing them to use the high positions to ambush attacking forces below.
But they faced internet shutdowns by the Taliban and supply roads were blocked.
Previously, Panjshir’s fighters melted away in the face of advancing forces, hiding in canyons off the main valley, then launching guerilla raids.
But the Taliban have been emboldened by their sweeping victories across the rest of the country, where they seized an enormous arsenal of weapons and military kit that the now-departed US provided to the defeated Afghan army.
The NRF is led by Ahmad Massoud – the son of Ahmad Shah Massoud – as well as ex-officials who fled to the valley from the toppled government, including former vice-president Amrullah Saleh, a fierce Taliban critic.
It is unclear where those senior Panjshir leaders are now based.
But Mr Massoud, in an audio message Monday after the Taliban declarations of victory, called for Afghans to “rise up”.
The Panjshir – mainly inhabited by ethnic Tajik people – has immense symbolic value in Afghanistan as the area that has resisted occupation by invaders in the past.
The Taliban warned on Monday that anyone who “tries to start an insurgency” against them “will be hit hard”.
But Mr Massoud’s message in reply was one of fighting bravado.
“For those who want to take up arms, we are with you,” he said in a call for resistance.
“For those who will resort to protest, we will stand next to you.”