Taliban fighters have launched an assault on the northeastern province of Panjshir in a bid to crush resistance fighters who recently captured three districts in the area.
A source familiar with the Taliban’s military operation confirmed to the Telegraph that military commanders had given the order to attack.
In a post on its Arabic-language Twitter account, the Taliban said “hundreds” of fighters were heading to Panjshir after peace talks with the rebels failed.
If the Taliban manages to seize control of Panjshir, a narrow and near-impenetrable valley which lies 100 miles north of Kabul, it would be the first time they have done so.
The valley was never captured during the Taliban’s first reign, thanks to the resistance of legendary warlord Ahmad Shah Massoud, who was backed by the West.
The assault began after Afghan resistance fighters recaptured three districts from the Taliban in northern provinces, which has created a dilemma for the United States over whether it should offer military support.
In the first major clashes since Afghanistan fell to the Taliban, the rebels claimed to have driven the insurgents away from the Pul-e-Hesar, Deh-e-Salah and Bano districts.
The fighters, who are believed to be remnants of Afghan security forces, also said they had killed at least 30 Taliban soldiers and captured a dozen others.
The Taliban offered to negotiate with the rebels about a political agreement before beginning the assault, said Russia’s ambassador to Kabul.
US officials are said to be “wary” of supporting the rebels because Washington is relying on the Taliban to secure Kabul airport as evacuations continue.
Any perceived US support for the rebels could jeopardise a fragile agreement with Taliban leaders who are allowing Americans safe passage to Kabul, US military officials told the New York Times.
The New York Times added that the resistance fighters had not made any requests for US military support, such as airstrikes, and that none had been offered publicly by the US government.
The Afghan army has all but collapsed in the face of the Taliban’s lightning-speed takeover over the country, though several thousand soldiers remain on the battlefield.
Many have moved to the northeastern Panjshir valley where they continue the fight. Among them is former Afghan vice-president Amrullah Saleh, who claims to be the leader of a unified anti-Taliban movement.
Over the weekend, the Daily Telegraph interviewed one of the rebel commanders, Ahmad Massoud, the Sandhurst-trained and Kin’gs College London-educated son of the fighter who held the valley in the 1990s.
“It is going to be a very bloody and hard war because the people of Panjshir are going to fight to the very last breath,” he said.
According to the Pentagon, the remnants of the Afghan army are not operating as “functional” units and there are doubts that they will prove to be an effective resistance movement.
Analysts say low morale – which was devastated by the US withdrawal of forces – was a key factor in so many Afghan troops surrendering their posts and weapons to Taliban fighters.
The removal of US air support from Afghanistan in particular is said to have convinced Afghan soldiers that they stood no chance against the advancing insurgents.
Speaking to the Daily Telegraph, analysts with significant experience of fieldwork in Afghanistan said that so-called surrender pacts also played a “huge role” in the takeover.
Taliban fighters have been giving out “pocket money” to disillusioned troops as an incentive to desert, while checkpoints on the Iran-Afghanistan border alone have secured around $40m (£29m) per year in tax income.
Graeme Smith, the co-author of a major new report examining the Taliban’s revenue streams, said surrender pacts had allowed the insurgents to gain popularity and eliminate Afghan army resistance in one blow.
“It’s very traditional for the Taliban and it’s how they swept to power the first time around,” said Mr Smith, an author and former political affairs officer for the United Nations in Afghanistan
“They were exceptionally eager to make everyone understand that if you surrender you’ll be treated humanely.”
Bribes were often used to persuade desertion, Mr Smith added, and in some cases they were also offered to homesick, underpaid Afghan troops as a “humane gesture.”
The money was extremely tempting for soldiers who were posted to checkpoints hundreds of miles away from their homes as it would allow them to pay for a ride home.
There was also an element of theatre to the surrender deals, with the Taliban publishing footage online of soldiers abandoning their posts.