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.
The border post dividing the Afghan town of Spin Boldak in Kandahar province on one side and the Pakistani town of Chaman on the other, is the second busiest crossing between the countries.
According to BBC correspondent Lyse Doucet, the crossing would be a major prize – symbolically and strategically – if the Taliban continue to hold it. It would give them significant customs revenue from the trade which flows back and forth and provide direct access to areas in Pakistan, where Taliban leaders and fighters are known to have been based for many years, she says.
The BBC has not been able to independently verify the reports but Pakistani officials confirmed the Taliban had taken the post. Journalists and the public have been told not to approach the border from the Pakistan side, and there is an urgent security meeting under way there, our reporter in Quetta has been told.
In a statement, the Taliban’s Zabihullah Mujahid told residents and traders their “security is guaranteed”.
But Afghan interior ministry spokesman Tareq Arian told the AFP news agency that while there had been “some movements near the border… security forces have repelled the attack”.