The Taliban captured Afghan provincial capitals Sar-e-Pul and Kunduz on Sunday, although some fighting was reported to be continuing.
This comes after the insurgents seized control of the capitals of Jowzjan and Nimruz provinces in the previous two days.
The battle for Kunduz, a city of 270,000 people and capital of northern Kunduz province, had raged since Saturday evening, residents told The National.
Local officials said the cities had fallen and the Taliban posted videos purporting to show the group's fighters inside government buildings in Kunduz.
“The fighting started last night, the Taliban took control of the areas around the city centre one by one. By morning they had breached the borders,” said Kunduz resident Mohammad Wahid, 24.
The extremist group had captured the central prison in Kunduz and freed the prisoners, he said.
“Only the airport is still under government control. The rest is with the Taliban and they are destroying a lot of the market area,” he said.
A local provincial leader said Afghan forces were cornered at the airport.
Similar reports were shared by a journalist from Sar-e-Pul, capital of northern Sar-e-Pul province.
“The province has completely fallen to the Taliban. They have taken over the governor's building. Only the airport and police chief’s headquarters remain with the government,” Rahima Gul, who requested her name be changed, told The National.
Ms Gul said the city was in “complete chaos".
"The battle is still going on with the Afghan forces, but some of the Afghan forces have surrendered to the Taliban,” she said.
Afghan forces have been conducting air strikes, residents in the provinces said.
But Ms Gul said it was difficult for Afghan forces to attack the Taliban because their fighters had entered homes and were attacking from there.
“I could hear the planes and helicopters roaming around the city, but they couldn't attack. The Taliban are hiding in people’s houses and it will cause a lot of civilian casualties,” she said.
Kunduz, the sixth largest city of Afghanistan, was a significant government stronghold, but has never recovered from being seized by the Taliban in 2015 and 2016.
“The context of Kunduz is that the province’s security never really improved,” said Andrew Watkins, senior Afghanistan analyst for think tank Crisis Group.
“Ever since the Taliban seized it in 2015, they’ve kept the city besieged and dominated large parts of the province.
“In that sense, it’s not as surprising that Kunduz fell. The Taliban have been entrenching themselves and mounting large assaults on the city for years."
The city is an important trade station and connects other parts of the country to the north-east.
“Until the government recovers Kunduz, it can’t recover border crossings into Tajikistan, so there is a geopolitical significance,” Mr Watkins said.
“It’s also very difficult to reach the rest of the north-east without going through Kunduz. It cuts a whole region off, by road.”
An Afghan security forces spokesman said the government had not given up in the city. He told Reuters that “extremely [heavy] fighting is going on".
Kunduz is the most significant gain for the insurgents since they launched an offensive in May, as foreign forces began the final stages of their withdrawal from the country.
On Friday, the Taliban seized their first provincial capital, Zaranj, in south-western Nimruz province, and followed it up a day later by taking Sheberghan in northern province Jowzjan.
In recent days, there has also been heavy fighting in other parts of the country, including Herat, capital of western Herat province, near the border with Iran, and Lashkar Gah, capital of Helmand province, in the south-west.
The pace of Taliban advances has caught government forces flat-footed, but there was some respite late on Saturday when US warplanes bombed Taliban positions in Sheberghan.
“US forces have conducted several air strikes in defence of our Afghan partners in recent days,” Maj Nicole Ferrara, a Central Command spokeswoman, told AFP in Washington.
The surge in violence has forced many residents to flee Kunduz, Mr Wahid said.
“The city is burning and people are afraid that the killings and rapes that happened in Kandahar, Spin Boldak and other places where Taliban took over, will happen here,” he said.
He said he was looking for an opportunity to take his family out of the besieged city.
“I have lost my business. The bullets are coming from both sides. We don’t know which way to go, and we are stuck in the front lines. It is finished for Kunduz," he said.
Ms Gul, who has escaped Sar-e-Pul, described similar scenes.
“The situation is very bad in Sar-e-Pul. I saw kids crying, women were carrying their belongings and leaving the city. It was a very disturbing image," she said.