The fall of Zaranj is a heavy blow to security forces, pictured here patrolling the western city of Herat
The Taliban in Afghanistan say they have captured a prison in the northern province of Jawzjan and freed all of the prisoners.
Video on social media shows hundreds of inmates leaving the prison in the city of Sheberghan on Saturday after the insurgents launched an attack.
The Taliban have taken control of the city, making it the second regional capital to fall to the militants.
It is a major blow to security forces, as battles escalate across the country.
Sheberghan is a stronghold of the former Afghan vice-president, Abdul Rashid Dostum, whose supporters have been leading the fight against the insurgents.
Local media reports that 150 people travelled to the city to help Afghan forces fight the Taliban.
The Taliban first took control of the governor’s compound on Friday during intense fighting, before it was retaken by Afghan security forces.
However, the region’s council chief, Babur Eshchi, told the BBC the militants were now in control of the whole city, except an army base, where fighting was still going on.
Violence is also raging in other parts of the country, as the Taliban continue to make rapid advances, displacing thousands of civilians.
The city of Zaranj, in Nimroz province, was the first regional capital to fall to the insurgents on Friday.
Other provincial capitals under pressure include Herat in the west, and the southern cities of Kandahar and Lashkar Gah.
A map showing who’s in control of Afghanistan
The Afghan military says dozens of Islamist fighters, including some senior commanders, have been killed in Lashkar Gah, however the Taliban have denied the military’s version of events.
The US and UK governments have urged its citizens to leave immediately because of the worsening security situation.