In this photo taken on September 27, 2020, a policeman keeps watch outside an outpost set up against Taliban fighters at Aziz Abad village in Maiwand district of Kandahar province, Afghanistan. Photo: AFP

The Taliban said Wednesday they had captured the strategic border crossing of Spin Boldak on the frontier with Pakistan, continuing sweeping gains made since foreign forces stepped up their withdrawal from Afghanistan.

The situation on the ground could not immediately be verified, with Afghanistan's interior ministry denying the report even as social media was abuzz with pictures of Taliban fighters looking relaxed in the frontier town.

Spin Boldak is the latest in a string of border crossings and dry ports seized by the Taliban in recent weeks, with the insurgents looking to choke off much-needed revenue from the government in Kabul while also filling their own coffers.

As Kabul's grip over the country appeared to further loosen, former US president George W. Bush slammed current White House incumbent Joe Biden's decision to withdraw all troops.

Civilians were being left to be "slaughtered" by the Taliban, Bush told German broadcaster Deutsche Welle on Wednesday.

"This is a mistake... They're just going to be left behind to be slaughtered by these very brutal people, and it breaks my heart," he said.

The Bush administration launched the US invasion into Afghanistan in 2001 that toppled the Taliban government following the September 11 attacks on US soil by Al Qaeda militants. 

Along with his key security advisors, Bush was later blamed for a series of miscalculations in Afghanistan that led to the revival of the Taliban movement. 

The seizure of the vital border crossing with Pakistan follows days of heavy fighting across Kandahar province, where the government was forced to deploy commando fighters to prevent the fall of the provincial capital even as the insurgents inched closer to taking the frontier.

In a statement, insurgent spokesman Zabihullah Mujahid assured traders and residents there that their "security is guaranteed."

But Afghan officials insisted they were still in control.

"The terrorist Taliban had some movements near the border area... The security forces have repelled the attack," interior ministry spokesman Tareq Arian told AFP.

Residents disputed the government's claims. 

"I went to my shop this morning and saw that the Taliban are everywhere. They are in the bazaar, in police HQ and custom areas. I can also hear the sound of fighting nearby," said Raz Mohammad, a shopkeeper who works near the border.

The border crossing is one of the most strategically valuable for the Taliban. 

Balochistan is a favored destination for fighters regularly heading for medical treatment and hosts many of their families.