The Taliban are reported to have raised their flag above a key border post between Afghanistan and Pakistan, and claim it is now under their control.
Videos being shared on social media show the white flag fluttering above the Spin Boldak crossing near Kandahar.
Afghan officials have denied the post has fallen, although pictures on social media show the militants chatting to Pakistani border guards.
The BBC has been told the Taliban took the border crossing with no resistance.
In recent weeks, the militants have made rapid advances across the country, seizing a series of border posts from Afghan forces, including crossings with Iran, Tajikistan and Turkmenistan. It comes as the US withdraws its forces from Afghanistan ahead of an 11 September deadline set by President Joe Biden.
The Taliban - a fundamentalist Islamist militia who were pushed out of power by the US invasion 20 years ago - have also seized control of a number of key roads as they seek to cut off supply routes to major cities.