Footage shows the scale of the assault in Herat as Taliban fighters rushed past the Great Mosque, ITV News Global Security Editor Rohit Kachroo reports
The insurgents have taken over 12 of Afghanistan's 34 provincial capitals in the last week.
The latest conquests on Thursday will further squeeze the country's government as the US army begins to withdraw its troops from the country.
The UK will be sending 600 troops into Afghanistan to help evacuate embassy staff and British nationals, Defence Secretary Ben Wallance announced on Thursday.
The additional military support will arrive in Kabul over the coming days.
The embassy in Kabul, which will be reduced to a core team, will be relocated outside of the city to a safer place in the green zone.
Sir Laurie Bristow, the UK’s Ambassador to Afghanistan, will lead a smaller team in Afghanistan and will focus on helping Brits leave the country and helping Afghans relocate to the UK.
Mr Wallace said: “I have authorised the deployment of additional military personnel to support the diplomatic presence in Kabul, assist British nationals to leave the country and support the relocation of former Afghan staff who risked their lives serving alongside us.
“The security of British nationals, British military personnel and former Afghan staff is our first priority. We must do everything we can to ensure their safety.”
These are the key cities that have been captured in Afghanistan on Thursday and where they sit on the map:
Last Friday, the UK government changed travel advice to recommend that all British nationals leave Afghanistan as soon as possible.
The US will also be sending an additional 3,000 troops into Afghanistan to help evacuate embassy staff, officials announced on Thursday.
The Pentagon, which is withdrawing its troops from Afghanistan, had kept about 650 troops in the country to support US diplomatic security, including at the airport.
The move suggests both countries are becoming less confident in the Afghan government's ability to hold onto the country's capital.
In Kandahar, the Taliban seized the governor's office and other buildings on Thursday night, witnesses said. The governor and other officials reportedly fled, catching a flight to Kabul.
The Taliban had earlier attacked a prison in the city and freed inmates inside, officials said.
Taliban fighters rushed past the Great Mosque in Herat, which dates to 500 BC and was once a spoil of Alexander the Great, and seized government buildings.
Witnesses described hearing gunfire at one government building while the rest of the city fell silent under the insurgents' control. And Taliban fighters once-detained at Herat's prison are now seen freely moving on the streets.
The US and others overestimated the Afghan government and its military and underestimated the Taliban, Rohit Kachroo explains
Herat had been under attack for two weeks before Taliban fighters finally broke through the city's defensive lines.
Afghan lawmaker Semin Barekzai acknowledged the city's fall to the Taliban, saying some officials had escaped.
On the same day, the Taliban captured Ghazni city, which cuts off a crucial highway linking the Afghan capital with the country's southern provinces.
Which provincial capitals have fallen to the Taliban so far?
Zaranj - captured on August 6 - The capital of Nimruz province in southwestern Afghanistan is linked by highways with Lashkar Gah to the east, Farah to the north and the Iranian city of Zabol to the west.
Sheberghan - captured on August 7 - The capital of the Jowzjan province in northern Afghanistan was strategic because it was the stronghold of US-allied Uzbek warlord Rashid Dostum, whose militias are among those resurrected to aid the Afghan National Security and Defence Forces.
Sar-e-Pul - captured on August 8 - The city is the capital of the homonymous province Sar-e-Pul and is located in the north.
Taloqan - captured on August 8 - The capital of Takhar province in the north east lies next to Kunduz and had particular significance for the anti-Taliban northern alliance fighters who joined the US-led coalition to oust the religious militia in 2001.
Kunduz - captured on August 8 - Kunduz, to the north and the capital of the homonymous province, is a strategic crossroads with good access to much of northern Afghanistan as well as the capital, Kabul, about 200 miles away. On August 9, the Taliban took over Kunduz airport, which is also one of seven key military bases, representing a major setback for the government forces.
Aybak - captured on August 9 - The capital of Samangan in the north was once known to be one of the safest provinces in Afghanistan, with a minimal Taliban presence.
Pul-e-Khumri - captured on August 10 - The capitol of Baghlan province, in the north, is 140 miles north of Kabul and gives insurgents control of a strategic road junction linking Kabul to the north and west.
Fayzabad - captured on August 11 - The city in Badakhshan province is in northeast Afghanistan.
Farah - captured on August 11 - Capital of Farah province in western Afghanistan
Ghazni - captured on August 12 - The capture of Ghazni, in southeastern Afghanistan, cuts off a crucial highway linking the capital with the country's southern provinces, which are also under attack. It could complicate resupply and movement for government forces.
Herat - captured on August 12 - In Herat province and west of the country, the capital is Afghanistan's third-largest city and a strategic provincial capital near Kabul.
Kandahar - captured on August 12 - Kandahar is the second largest city after Kabul and located in the south of the country.
Which areas could fall in coming days?
Fighting is also raging in Lashkar Gah, one of Afghanistan’s largest cities in the Taliban heartland of Helmand province. Surrounded government forces hoped to hold onto the southern provincial capital.
Taliban militants patrol Ghazni in utility trucks after the city fell into their hands
More than 200,000 children in Afghanistan have been forced to flee their homes as the country's conflict intensifies, UNICEF has said.
The global children's charity warned of a "rapid escalation of grave violations" against youngsters.
UNICEF found around 400,000 Afghans have become internally displaced. Over half of these are children and four million pupils are missing school.
Afghan children at a camp for the internally displaced in the city of Kandahar (Credit: Unicef)
While Kabul itself isn’t directly under threat, the latest US military intelligence assessment suggests it could come under insurgent pressure within 30 days.
The US also implied that if current trends hold, the Taliban could gain full control of the country within a few months.
The Afghan government may eventually be forced to pull back to defend the capital and just a few other cities.
The loss of Kandahar, Herat and Ghazni tightens the grip of a resurgent Taliban estimated to now hold some two-thirds of the nation.
The onslaught in Afghanistan renews questions about where the over $830 billion (around £600 billion) spent by the US Defense Department on fighting, training those troops, and reconstruction efforts went, especially as Taliban fighters ride on American-made utility trucks.
It also raised fears that the Taliban would turn back the clock on the country and reimpose a brutal regime. Already there are reports of repressive restrictions on women and revenge killings.
Afghan president Ashraf Ghani is trying to rally a counteroffensive relying on his country’s special forces, the militias of warlords and American airpower ahead of the US and NATO pull-out at the end of the month.
On Thursday, Taliban militants raised their white flags imprinted with an Islamic proclamation of faith over the city of Ghazni, just 130 kilometres southwest of Kabul.
Mohammad Arif Rahmani, a lawmaker from Ghazni, told the Associated Press the city had fallen to the insurgents. Ghazni provincial council member Amanullah Kamrani echoed this, but added that the two bases outside of the city remain held by government forces. Already, the Taliban’s weeklong blitz has seen the militants seize nine other provincial capitals around the country. Many are in the country’s northeast corner, pressuring Kabul from that direction as well.