The Taliban took prisoner and then killed Rohullah Saleh, brother of former Afghan Vice President Amrullah Saleh, media reports said.
A source close to the resistance forces told Russian news agency Sputnik that Rohullah had died a few days ago fighting against the Taliban.
This comes a few days after reports emerged that Amrullah Saleh, who was residing at the Panjshir, has fled to Tajikistan.
Meanwhile, the fate of Ahmad Massoud, the resistance leader in the Panjshir Valley remains unknown.
The Panjshir Valley lies in the Hindu Kush mountains, approximately 90 miles north of Kabul.
After the fall of Kabul on August 15, Panjshir province remained the only defiant holdout where resistance forces led by Ahamd Masoud, the son of late former Afghan guerrilla commander Ahmad Shah Massoud, were fighting the Taliban.
Panjshir has been the stronghold of the National Resistance Front, led by Ahmad Massoud and Amrullah Saleh, who had declared himself caretaker president.