Afghan President Ashraf Ghani has flown to the besieged northern city of Mazar-i-Sharif to rally his beseiged forces.

Taliban fighters have now taken more than a quarter of the country's provincial capitals in less than a week.

Mr Ghani arrived in Mazar as the Taliban captured Faizabad overnight, making it the ninth city to be overrun since Friday last.

He plans "to check the general security in the northern zone", according to a statement released by the palace.

The Afghan leader was also likely to hold talks with Mazar's long-time strongman Atta Mohammad Noor and warlord Abdul Rashid Dostum about the defence of the city, as Taliban fighters inched closer to its outskirts.

The loss of Mazar would be a catastrophic blow to the Kabul government and represent the complete collapse of its control over the north - long a bastion of anti-Taliban militias.

Hours before President Ghani arrived, pictures posted on official government social media accounts showed Dostum boarding a plane in Kabul, along with a contingent of commandos, en route to Mazar.

Fighting in Afghanistan's long-running conflict has escalated dramatically since May, when the US-led military coalition began the final stage of a withdrawal set to be completed before the end of the month.

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Further to the east of Mazar in Badakhshan's capital Faizabad, a resident told reporters that security forces had retreated after days of heavy clashes.

The insurgents also confirmed in a social media post that their fighters were in control of the city.

As the Taliban routed government forces, US President Joe Biden gave no hint of delaying his deadline to withdraw all American troops by 31 August, instead urging Afghan leaders to "fight for themselves" on Tuesday.

"I do not regret my decision" to withdraw US troops after two decades of war, he told reporters in Washington.

And as fighting raged, US diplomats were desperately trying to breathe life back into all but dead talks between the Afghan government and Taliban in Doha, where Washington's special envoy Zalmay Khalilzad was pushing the Taliban to accept a ceasefire.

Mr Biden has stressed that Washington would continue to support the Afghan security forces with air strikes, food, equipment and money for salaries.

"They have got to want to fight. They have outnumbered the Taliban," he said.

The Taliban have appeared largely indifferent to peace overtures, and seem intent on a military victory to crown a return to power after their ouster 20 years ago in the wake of the 11 September attacks.

After conquering most of the north, the Taliban have now set their sights on region's biggest city, Mazar-i-Sharif - long a linchpin for the government's control of the area - after capturing Sheberghan to its west, and Kunduz and Taloqan to its east.

Government forces are also battling the hardline Islamists in Kandahar and Helmand, the southern Pashto-speaking provinces from where the Taliban draw their strength.