China evacuated 210 nationals from Afghanistan as US troops stepped up their withdrawal from the war-torn nation, the airline that carried out the emergency chartered flight said.

A Xiamen Airlines flight departed the capital Kabul for the city of Wuhan, Hubei province, on July 2 carrying Chinese citizens who had been stranded in Afghanistan, it said in a Thursday social media post. 

Among them were 22 people who were later confirmed to be infected with coronavirus, despite the airline taking "top-notch epidemic prevention measures" during the flight and upon landing, it added. 

The Chinese foreign ministry confirmed that recent returnees included coronavirus patients and called on all nationals to leave Afghanistan, without revealing further specifics of the evacuation flight. 

"In order to ensure the safety of Chinese citizens in Afghanistan, the Chinese government has reminded citizens in Afghanistan to leave the country as soon as possible and provided necessary assistance," its consular affairs department said in a Wednesday social media post. 

Chinese health authorities reported 25 new imported coronavirus cases in Hubei province on Wednesday, 22 of which were from the Kabul flight.

The flight was organised by the Chinese government, reported the state media tabloid Global Times.

President Biden has vowed that US troops will leave Afghanistan by August 31, after two decades of bloody conflict in the region. 

With the Taliban having routed much of northern Afghanistan in recent weeks, fears are growing over the government's waning control of a handful of provincial capitals.

In recent weeks, Beijing has harshly criticised what it sees as a hasty and chaotic withdrawal by Washington.

"The US disregards its responsibilities and duties and withdraws troops from Afghanistan hastily, dumping the mess and war on the Afghan people and countries in the region," foreign ministry spokesman Wang Wenbin told a routine briefing Friday.

"The US, as the original culprit of the Afghan issue, bears unavoidable responsibility for the current situation in Afghanistan."

Chinese foreign minister Wang Yi is set to discuss the deteriorating Afghan security situation with counterparts from Russia, India, Pakistan and numerous Central Asian countries at a meeting of the Shanghai Cooperation Organisation next week.

The evacuation flight was hailed as a patriotic victory by Chinese social media users, with related trending hashtags gaining more than 300 million views on Weibo. 

"This route is not easy to fly even once, we cannot leave a single compatriot behind," the pilot was quoted as saying in the Xiamen Airlines post. 

Govt to rush troops to border
The Taliban claimed to be in control of 85 percent of Afghanistan as authorities Saturday prepared to retake a key border crossing seized by the insurgents in a sweeping offensive launched as US troops pull out of the war-torn nation.

Hours after President Joe Biden issued a staunch defence of the US withdrawal on Thursday, the Taliban said its fighters had seized two crossings in western Afghanistan -- completing an arc of territory from the Iranian border to the frontier with China.

In Moscow, a delegation of Taliban officials said they controlled about 250 of Afghanistan's nearly 400 districts -- a claim impossible to independently verify, and disputed by the government.

Taliban spokesman Zabihullah Mujahid separately told AFP their fighters had captured the border town of Islam Qala on the Iranian frontier and the Torghundi crossing with Turkmenistan.

Herat governor spokesman Jilani Farhad said the authorities were preparing to deploy new troops to retake Islam Qala port, the biggest trade crossing between Iran and Afghanistan.

"Reinforcements have not been sent to Islam Qala yet. They will be sent there soon," he told AFP.

The Afghan government has repeatedly dismissed the Taliban's gains as having little strategic value, but the seizure of multiple border crossings along with mineral-rich areas will likely fill the group's coffers with several sources of new revenue.

Biden said the US military mission would end on August 31 -- nearly 20 years after it began -- but he admitted it was "highly unlikely" Kabul would be able to control the entire country.

"The status quo is not an option," Biden said of staying in the country. "I will not send another generation of Americans to war in Afghanistan."

With the Taliban having routed much of northern Afghanistan in recent weeks, the government holds little more than a constellation of provincial capitals that must largely be reinforced and resupplied by air.

The air force was under severe strain even before the Taliban's lightning offensive overwhelmed the government's northern and western positions, putting further pressure on the country's limited aircraft and pilots.

Biden said the Afghan people alone should determine their future, but he acknowledged the uncertainty about what that would look like.

Asked if a Taliban takeover was inevitable, the president said: "No, it is not."

But, he admitted, "the likelihood there is going to be one unified government in Afghanistan controlling the whole country is highly unlikely".

- US exit 'positive step' -
Afghan commandos clashed with the insurgents this week in a provincial capital, with thousands of people fleeing Qala-i-Naw in northwest Badghis province.

On Friday the Afghan defence ministry said government forces had "full control" of the city, but a local official said on Saturday the insurgents had attacked the city again during the night.

Ismail Khan, a veteran warlord whose militia helped US forces topple the Taliban in 2001, vowed to back government forces in fighting against the insurgents.

"We will soon go to the front lines and with the help of God change the situation," Khan told reporters in the western city of Herat.

The Taliban have been emboldened by the troop withdrawal and -- with peace talks in Doha deadlocked -- appear to be pressing for a full military victory.

Still, on Thursday, Suhail Shaheen, who is also a member of the Taliban negotiating team, insisted the insurgents were seeking a "negotiated settlement".

US Defense Secretary Lloyd Austin called for international pressure to force a deal.

"The security situation in Afghanistan only argues more for international pressure to have a negotiated political settlement," Austin said in a tweet on Friday.

"The entire world can help by continuing this push."

The Pentagon chief did not specify which countries he was urging to help, but Pakistan is widely believed to have significant influence over the insurgents.