The US military has said it could slow down its withdrawal from Afghanistan in light of recent battlefield victories by the Taliban.
Pentagon spokesman John Kirby said the deadline for a full withdrawal by 11 September was still in place, but the pace may change.
Officials said last week that the withdrawal is about half-way done.
Afghanistan has seen increasing violence as the US and Nato prepare to pull out their remaining troops.
“The situation in Afghanistan changes as the Taliban continue to conduct these attacks and to raid district centres as well as the violence, which is still too high,” the Pentagon spokesman said.
“If there needs to be changes made to the pace, or to the scope and scale of the retrograde, on any given day or in any given week, we want to maintain the flexibility to do that.
“We’re constantly taking a look at this, every single day: what’s the situation on the ground, what capabilities do we have, what additional resources do we need to move out of Afghanistan and at what pace.
“All of these decisions are literally being made in real time.”
The UN special envoy on Afghanistan, Deborah Lyons, said the Taliban had taken more than 50 of 370 districts since May, warning that increased conflict “means increased insecurity for many other countries, near and far”.
“Those districts that have been taken surround provincial capitals, suggesting that the Taliban are positioning themselves to try and take these capitals once foreign forces are fully withdrawn,” she told the UN Security Council on Tuesday.
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.
The hardline Islamist group say they have control of the whole province, with only the provincial capital – also named Kunduz – retained by the government. But the defence ministry 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 General 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.
US President Joe Biden says the American pull-out 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 that al-Qaeda was still “heavily embedded” within Taliban militants in Afghanistan, however.
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.
Mr Biden has pledged that the US will continue to support Afghanistan after pulling troops out, but not “militarily”.
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 is 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.