The first US troops are arriving in Afghanistan to help diplomatic staff and others leave, as many countries scramble to evacuate workers and citizens amid a rapid Taliban advance.
On Friday, militants captured Pul-e-Alam, capital of Loghar province, just 80km (50 miles) from the capital Kabul.
The UN chief said the situation was spinning out of control with devastating consequences for civilians.
More than 250,000 people have been forced to leave their homes so far.
The Taliban advance comes as US and other foreign troops withdraw after 20 years of military operations. The fighting has raised fears that gains in human rights made since the militants were ousted from power in 2001 could be swiftly reversed.
Life under the Taliban in the 1990s saw women forced to wear the all-covering burka, education restricted for girls over 10, and brutal punishments including public executions.
Also on Friday, the Taliban seized the country’s second-largest city Kandahar and the nearby city of Lashkar Gah, as well as Herat in the west. They now control about a third of Afghanistan’s provincial capitals.
Pentagon spokesman John Kirby called the recent advances “deeply concerning”, but downplayed any suggestion that Kabul was under imminent threat from the group.
Most of the 3,000 troops being sent to help evacuate US diplomatic staff will arrive by the end of the weekend. The US intends to airlift thousands of people a day out of Kabul.
The latest US intelligence assessment suggests the militants could try to advance on the national capital within 30 days.
The US embassy there informed staff that an incinerator and other tools were available to destroy sensitive material, including documents and equipment such as flags that could be used in propaganda.
The UK, which is sending 600 troops to aid the evacuation of British nationals and former Afghan staff, said staffing at its embassy would be reduced to an absolute minimum – as did Germany.
Denmark and Norway are closing their embassies altogether.
Agencies