The Taliban raised their flag over the Afghan presidential palace on the 20th anniversary of the 9/11 attacks, as news emerged that the regime’s fighters had killed the brother of the country’s former vice-president at a checkpoint in Panjshir province.

Rohullah Azizi, the brother of former vice-president and anti-Taliban resistance leader Amrullah Saleh, was travelling in his car on Thursday when he and his driver were shot dead at a Taliban checkpoint, his nephew said on Saturday.

Shuresh Saleh said on Saturday that it was unclear where his uncle, an anti-Taliban fighter, was headed when the Taliban caught him. He said phones were not working in the area.

A message left with a Taliban spokesman on Saturday was not immediately returned.

Amrullah Saleh has declared himself the legitimate acting president of Afghanistan, and has been leading forces resisting the Taliban in Panjshir. Afghanistan’s new rulers have said they are in control of Panjshir but this is disputed by the National Resistance Front, which says it is continuing to fight back. Videos circulating on social media purportedly show the Taliban opening fire on anti-Taliban fighters arrested in Panjshir.

As the US and the world marked the anniversary of the September 11 attacks on Saturday, the Taliban’s flag-raising ceremony marked the official start of the all-male, all-Taliban government, a spokesman said.

The white banner bearing a Quranic verse was hoisted by Mullah Mohammad Hassan Akhund, the prime minister of the Taliban interim government, said Ahmadullah Muttaqi, multimedia branch chief of the Taliban’s cultural commission.

The milestone anniversary came just weeks after the chaotic US withdrawal from Afghanistan and the return to power of the Taliban, which had sheltered the al-Qaida terror network founded by Osama bin Laden that carried out the attacks.

The US president, Joe Biden, speaking unexpectedly during a visit to the Pennsylvania site of one of the 9/11 plane crashes on Saturday, again defended the widely criticised withdrawal, saying the US could not invade every country where al-Qaida is present.

“Could al-Qaida come back [in Afghanistan]?” he asked in an exchange with reporters outside a Shanksville fire station. “Yeah. But guess what, it’s already back other places.

“What’s the strategy? Every place where al-Qaida is, we’re going to invade and have troops stay in? C’mon.”

Biden said it had always been a mistake to think Afghanistan could be meaningfully united, and that American forces had achieved their central mission when a special forces team killed the al-Qaida founder Osama bin Laden on 2 May 2011 in a compound in Pakistan.

On Saturday the Taliban orchestrated a march of veiled women that filled an auditorium at Kabul University’s education centre in a well-choreographed snub to the past 20 years of western efforts to empower women. In reality the Taliban have begun issuing harsh edicts that have hit women hardest, such as banning women’s sports. They have also used violence to stop protests where women have demanded equal rights.

The Associated Press and Agence France-Presse contributed to this report