The insurgents have taken more than 50 of 370 districts since May, UN special envoy Deborah Lyons told the Security Council, warning of “dire scenarios”.
She said increased conflict “means increased insecurity for many other countries, near and far”.
The US and Nato are still aiming for a complete troop pullout by 11 September.
However, Pentagon spokesman John Kirby said the situation remained “dynamic” and, although the Taliban gains had not changed the withdrawal, there was still the flexibility to alter its “pace and scope”.
The hardline Islamist group’s recent advances were the result of an “intensified military campaign”, Ms Lyons told the the 15-member UN Security Council in New York.
“Those districts that have taken surround provincial capitals, suggesting that the Taliban are positioning themselves to try and take these capitals once foreign forces are fully withdrawn.”
UN Afghan envoy Deborah Lyons
The Taliban also captured Afghanistan’s main border crossing with Tajikistan on Tuesday, officials said. The crossing stands in the northern province of Kunduz, where fighting has escalated in recent days.
Taliban fighters say they have control of the whole province, with only the provincial capital; also named Kunduz – retained by the government. But the defence ministry in Kabul said Afghan forces had recaptured some districts and operations were ongoing.
Kunduz city is strategically significant, and briefly fell to the insurgents in 2015 and again a year later; before being retaken both times by Nato-backed government forces.
Local media report that the Taliban have also seized large quantities of military equipment; and killed, wounded or captured dozens of troops. The group’s own casualty figures are unclear.
Afghan security forces said they would launch a massive offensive shortly to reclaim lost territory.
“You will soon witness our advances across the country,” said spokesman Gen Ajmal Shinwari.
US-led forces ousted the Taliban from power in Afghanistan in October 2001. The group had been harbouring Osama Bin Laden and other al-Qaeda figures linked to the 9/11 attacks in the US.
Joe Biden About The Matter
US President Joe Biden says the American pullout is now justified as US forces have made sure Afghanistan cannot again become a base for foreign jihadists to plot against the West.
A senior United Nations official warned last year, however, that al-Qaeda was still “heavily embedded” within Taliban militants in Afghanistan.
Afghan President Ashraf Ghani says government forces are fully capable of keeping insurgents at bay; but many believe the withdrawal could cast Afghanistan back into the grip of the Taliban.
Writing in the Washington Post on Tuesday, Pakistan’s Prime Minister Imran Khan said his country was willing to be a “partner for peace in Afghanistan” with the US, but would not host US bases.
He said Pakistan had previously made mistakes by choosing between warring parties in neighbouring Afghanistan; and pledged to work with anyone who enjoyed the confidence of the Afghan people.
Afghan leaders have long accused Pakistan of supporting the Taliban. The country’s co-operation seen as critical to US withdrawal goals.
Mr Khan said recently that he would “absolutely not” allow the CIA into Pakistan to conduct cross-border counter-terrorism missions against al-Qaeda, the Islamic State group or the Taliban.
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