Hours after President Joe Biden issued a staunch defence of the US withdrawal, the Taliban said its fighters had seized the 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.

Afghanistan's interior ministry spokesman Tareq Arian said efforts were under way to dislodge the insurgents from their latest positions.

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.

Hours earlier, Biden said the US military mission would end on August 31 -- nearly 20 years after it began -- having "achieved" its goals.

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' -

The Taliban, for their part, welcomed Biden's statement.

"Any day or hour that US and foreign troops leave earlier is a positive step," spokesman Suhail Shaheen told AFP.

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.

Hours later a group of Taliban fighters attacked a prison on the edge of Kandahar city, the capital of their former bastion of Kandahar province.

President Ashraf Ghani said the government could handle the situation, but admitted difficulties lay ahead.

"What we are witnessing is one of the most complicated stages of the transition," he said in a speech in Kabul on Thursday.

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, 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 between the Kabul government and the Taliban.

"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.

In Moscow, a Russian foreign ministry spokeswoman said the Taliban controlled about two-thirds of the Afghan-Tajik border as a delegation from the insurgents wound up a visit.

About "85 percent of Afghanistan's territory" was under the group's control, said Taliban delegate Shahabuddin Delawar, who added that the group was committed to preventing other jihadists from using Afghanistan as a base for their operations.

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