A Taliban flag flies in the main square of Kunduz city on Sunday. AP

The Taliban on Monday seized a sixth Afghan provincial capital in less than a week, overrunning the northern city of Aibak, its deputy governor and an insurgent spokesman said.

The Taliban "are in full control", Sefatullah Samangani from Samangan province told the media, shortly after a Taliban spokesman tweeted that all government and police installations had been "cleared.”

An official said on Monday that Taliban took control of another provincial capital in Afghanistan after Kunduz, the capital of Kunduz Province.

The city's fall was the latest in a weekslong, relentless Taliban offensive as American and NATO forces finalise their pullout from the war-torn country.

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According to Mohammad Noor Rahmani, the council chief of northern Sar-e Pul province, the Taliban overran the provincial capital after over a week of resistance by the Afghan security forces, after which the city of Sar-e Pul collapsed. The government forces have now completely withdrawn from the province, he said.

Several pro-government local militia commanders also surrendered to the Taliban without a fight, allowing the insurgents to gain control of the entire province, Rahmani added.

The city of Sar-e Pul joins three other provincial capitals now fully under Taliban control: Zaranj, the capital of western Nimroz province, the city of Shibirghan, the capital of northern Zawzjan province, and Taleqan, the capital of another northern province with the same name.

Several pro-government local militia commanders also surrendered to the Taliban without a fight.

The Taliban are also fighting on for control of the city of Kunduz, the capital of northern Kunduz province. On Sunday, they planted their flag in the city's main square, where it was seen flying atop a traffic police booth, a video obtained by The Associated Press showed.

A spokesman for the Taliban's political office told the media on Sunday that there is no agreement on a ceasefire with the Afghan government, and warned against further US intervention in Afghanistan.

The militants have ramped up their push across much of Afghanistan, turning their guns on provincial capitals after taking large swaths of land in the mostly rural countryside. At the same time, they have been waging an assassination campaign targeting senior government officials in the capital, Kabul.

Smoke rises from damaged shops after fighting between Taliban and Afghan security forces in Kunduz city. AP

Meanwhile, suspected Taliban fighters killed an Afghan radio station manager in Kabul and kidnapped a journalist in southern Helmand province, local government officials said on Monday, reporting the latest in a long line of attacks targeting media workers.

Gunmen shot Toofan Omar, the station manager of Paktia Ghag radio and an officer for NAI, a rights group supporting independent media in Afghanistan, in a targeted killing in the capital on Sunday.

The city of Sar-e Pul joins three other provincial capitals now fully under Taliban control: Zaranj, the capital of western Nimroz province, the city of Shibirghan, the capital of northern Zawzjan province, and Taleqan, the capital of another northern province with the same name.

On Saturday, Taliban fighters entered the capital of the northern Jawzjan province after sweeping through nine of 10 districts in the province. And the city of Kandahar, the provincial capital of Kandahar, also remains under siege.

As they rolled through provincial capitals, the Taliban issued an English language statement on Sunday saying that residents, government employees, and security officials had nothing to fear from them.

However, revenge attacks and repressive treatment of women have been reported in areas now under Taliban control.