- War in Afghanistan (2001-present)
image copyrightGetty Imagesimage captionThe US hopes to have all of its troops out of Afghanistan by 11 September, 2021
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.
Pentagon 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.
Over the last month, the Taliban have intensified their attacks and gained control of more than 30 districts. The hardline Islamist group has also seized large quantities of military equipment, according to local media, and killed, wounded or captured dozens of troops.
Afghan government spokespeople have denied that the districts have fallen to the Taliban, saying they were evacuated in a "tactical withdrawal". It is unclear how many Taliban have been killed or wounded.
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The Taliban say they have control of the whole northern province of Kunduz, with only the provincial capital retained by the government.
Police said the group had encircled the strategically important city, also named Kunduz, the Associated Press reported. A long-standing target, it briefly fell to the insurgents in 2015 before being retaken by Nato-backed government forces.
Afghan security forces continue to resist the Taliban push, and recaptured two districts in the north-eastern province of Takhar on Sunday.
image copyrightGetty Imagesimage captionAfghan security forces pictured in the strategic city of Kunduz
"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."
media captionTop US commander General Scott Miller reflects on Nato forces' time in Afghanistan ahead of its departure
US-led forces ousted the Taliban from power in Afghanistan in October 2001. The Taliban 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 now fully capable of keeping insurgents at bay, but many believe the withdrawal could cast Afghanistan back into the grip of the Taliban.
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Mr Biden has pledged that the US will continue to support Afghanistan after pulling troops out, but not "militarily".
image copyrightGetty Imagesimage captionAfghanistan's leaders say Afghan security forces are capable of keeping the Taliban's fighters at bay
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.