Kabul (AFP) -
The Taliban said Friday they had captured a key border crossing with Iran, hours after President Joe Biden issued a staunch defence of the US military withdrawal from Afghanistan.
The group have captured more than a third of the country's 400 districts since the US accelerated its final pullout in early May, with the insurgents now holding an arc of territory from the Iranian border to the frontier with China.
A government official said efforts were under way to recapture Islam Qala -- the main conduit for trade between Afghanistan and Iran -- as the insurgents continue to make sweeping gains across the country.
'All Afghan security forces including the border units are present in the area, and efforts are under way to recapture the site,' interior ministry spokesman Tareq Arian told AFP.
Taliban spokesman Zabihullah Mujahid said the crossing was 'under our full control'.
Hours earlier, Biden said the US military mission will end on August 31 -- nearly 20 years after it began -- having 'achieved' its goals.
But he also admitted that 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.'
After the Taliban routed much of northern Afghanistan in recent weeks, the government is holding little more than a constellation of provincial capitals that must be largely reinforced and resupplied by air.
Afghanistan's air force was already under severe strain before the Taliban's lightning offensive overwhelmed the government's northern and western positions, which will likely put further pressure on the country's limited aircraft and pilots.
Biden said the United States 'did not go to Afghanistan to nation-build' and that the Afghan people alone should determine their future.
But he acknowledged the uncertainty about what that future 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.'
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 and the insurgents have clashed this week in a provincial capital for the first time, with thousands of people fleeing Qala-i-Naw in northwest Badghis 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.
'Legitimacy is ours; God is with us.'
The Taliban have been emboldened by the troop withdrawal and with peace talks with the government deadlocked, appear to be pressing for a full military victory.
Still, on Thursday a member of the negotiating team in Doha insisted the insurgents were seeking a 'negotiated settlement'.
'We do not believe in monopoly of power,' Shaheen told AFP.