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Hundreds of Taliban fighters were heading to the rugged Panjshir valley on Sunday, the group said in a statement, raising fears of heavy fighting in the last stronghold of resistance to the militants.
"Hundreds of Mujahideen of the Islamic Emirate are heading towards the state of Panjshir to control it, after local state officials refused to hand it over peacefully," the group wrote on its Arabic Twitter account.
Panjshir valley is the home of Ahmad Massoud, the son of a locally renowned veteran of the Afghan-Soviet war, the late resistance commander Ahmad Shah Massoud, who fought during a bitter civil war with the Taliban after the departure of Russians in 1989.
Mr Massoud has formed the National Resistance Front, a political and military organisation in the valley that is opposed to the Taliban's rule.
Ali Maisam Nazary, a spokesman for Mr Massoud, said on Sunday that a force of 9,000 fighters had gathered in the valley and was preparing for "long term conflict".
Thousands of Afghan civilians and a smaller number of security forces fled to the valley following the sudden fall of Kabul on August 15, joined by Vice President Amrullah Saleh, who grew up in the valley and fought alongside Mr Massoud's forces.
Panjshir leaders defiant
In recent days, the National Resistance Front has been taking part in government formation talks with the Taliban, but any chance of agreement has seemed remote.
Last week, Mr Massoud told the Atlantic Council that "local militias" should be supported across the country to resist the Taliban. On Friday, he told The National that any clashes in Panjshir would lead "to a very bloody and hard war because the people of Panjshir are going to fight to the very last breath.”
"The conditions for a peace deal with the Taliban is decentralisation – a system that ensures social justice, equality, rights, and freedom for all," Mr Nazary told AFP.
Mr Saleh has been similarly defiant since the ousting of President Ashraf Ghani.
"I won't disappoint millions who listened to me. I will never be under one ceiling with Taliban," Mr Saleh tweeted as Kabul fell to the militants.
A bloody battle for Panjshir valley could prove a headache for the militants, former foes of Mr Massoud’s father, who led a band of Tajik locals defending the valley, firstly against the Soviets in the 1980s and later, against the Taliban during the Afghan civil war.
The Panjshir valley became infamous for Russian forces during the Soviet-Afghan war. Between 1980 and 1985, the Russians launched nine offensives to clear the valley of Massoud’s Mujahideen fighters.
Each offensive consisted of a full infantry division, around 15,000 men, backed by heavy bomber air support from the sprawling Bagram air base near Kabul, with Russian aircraft raining bombs onto the villages in the valley.
All but one of the offensives made progress – the fifth operation in 1982, where elite paratroopers managed to get into the valley through parallel gorges.
The short-lived Russian advance forced Massoud to press for a ceasefire with the Russians, but he used the time to regroup his forces and the violence began again.
The valley never fell, an ominous historical fact for the Taliban who possess a small number of captured aircraft that they are unlikely to be able to operate effectively, in contrast to the now defunct Soviet air force.
But the Taliban may have some advantages that the Russians lacked in a new struggle for the valley.
Many Russians sent to fight in the mountains of Afghanistan were conscripts who suffered poor morale. Russian infantry divisions also had extensive logistical requirements, a problem exacerbated by Afghanistan's rugged mountainous terrain and harsh climate.
By contrast, the Taliban also have smaller logistical requirements and may have more success rallying forces to the Panshir region.