Taliban fighters could isolate Afghanistan's capital in 30 days and possibly take it over within 90, a US defence official cited US intelligence as saying, as the resurgent militants made more advances across the country.

The official, speaking to Reuters on condition of anonymity on Wednesday, said the new assessment of how long Kabul could stand was a result of the Taliban's rapid gains as US-led foreign forces leave.

"But this is not a foregone conclusion," the official added, saying that the Afghan security forces could reverse the momentum by putting up more resistance.

The Islamists now control 65% of Afghanistan and have taken or threaten to take 11 provincial capitals, a senior EU official said on Tuesday. Faizabad, in the northeastern province of Badakhshan, on Wednesday became the eighth provincial capital to be seized by the Taliban.

Fighting was extremely intense in Kandahar city, a doctor based in southern Kandahar provice said. The city received scores of bodies of Afghan forces and some injured Taliban.

All gateways to Kabul, which lies in a valley surrounded by mountains, were choked with civilians fleeing violence, a Western security source said. It was hard to tell whether Taliban fighters were also getting through, the source said.

"The fear is of suicide bombers entering the diplomatic quarters to scare, attack and ensure everyone leaves at the earliest opportunity," he said.

The speed of the Taliban advance has shocked the government and its allies. The group, which controlled most of Afghanistan from 1996 to 2001, when it was ousted for harbouring al Qaeda chief Osama bin Laden after September 11, wants to defeat the US-backed government and reimpose strict Islamic law.

US State Department spokesman Ned Price said the attacks were against the spirit of a 2020 agreement.

The Taliban committed to talks on a peace accord that would lead to a "permanent and comprehensive ceasefire," Price said on Wednesday. "All indications at least suggest the Taliban are instead pursuing a battlefield victory."

"Attacking provincial capitals and targeting civilians is inconsistent with the spirit of the agreement," he said.

The United Nations said more than 1,000 civilians had been killed in the past month, and the International Committee of the Red Cross said that since August 1 some 4,042 wounded people had been treated at 15 health facilities.

The Taliban denied targeting or killing civilians and called for an independent investigation.

The group "has not targeted any civilians or their homes in any locality, rather the operations have been undertaken with great precision and caution," spokesperson Suhail Shaheen said in a statement on Wednesday.

Peace talks

The loss of Faizabad was the latest setback for the government of President Ashraf Ghani, who flew to Mazar-i-Sharif to rally old warlords to the defence of the biggest city in the north as Taliban forces closed in.

Ghani spent years sidelining the warlords as he tried to project the authority of his central government over wayward provinces.

US President Joe Biden said on Tuesday he did not regret his decision to withdraw and urged Afghan leaders to fight for their homeland.

Washington had spent more than $1 trillion over 20 years and lost thousands of US troops, and continued to provide significant air support, food, equipment and salaries to Afghan forces, he said.

The Afghans "need to determine ... if they have the political will to fight back and if they have the ability to unite as leaders to fight back," White House press secretary Jen Psaki said.

She declined to comment on the intelligence assessments that Kabul could be overtaken by the Taliban within 90 days, which were first reported by the Washington Post, but said the plan to withdraw troops by August 31 held.

A source familiar with the assessments said they paint a range of possible outcomes including a rapid Taliban takeover, an extended fight and a possible negotiated agreement between the Taliban and current government.

New generation

The Taliban advances have raised fears of a return to power of the hardline militants who formed in 1994 from the chaos of civil war.

A new generation of Afghans, who have come of age since 2001, fears the progress made in areas such as women's rights and media freedom will be squandered.

The State Department's Price said the United States was working to forge an international consensus behind the need for a peace accord. The Taliban have captured districts bordering Tajikistan, Uzbekistan, Iran, Pakistan and China, heightening regional security concerns.

He spoke as envoys from the United States, China, Russia and other countries met in Doha with Taliban and Afghan government negotiators in a bid to break a months-long deadlock in peace talks.

Pakistani Prime Minister Imran Khan said Taliban leaders told him earlier this year that they will not negotiate with the Afghan government as long as Ghani remains president.