KUNDUZ, Afghanistan: The Taliban tightened the noose over northern Afghanistan Sunday, capturing two more provincial capitals as they take their fight to the cities after seizing much of the countryside in recent months.

The insurgents have snatched up four provincial capitals in Afghanistan since Friday in a lightning offensive that appears to have overwhelmed government forces.

Kunduz and Sar-e-Pul in the north fell within hours of each other Sunday, lawmakers and residents in the cities confirmed, but not without fierce fighting.

Pro-Taliban social media accounts claimed the insurgents were also close to overrunning the provincial capital of Taloqan.

In Kunduz, one resident described the city as being enveloped in "total chaos".

"After some fierce fighting, the mujahideen, with the grace of God, captured the capital of Kunduz," the Taliban said in a statement.

"The mujahideen also captured Sar-e-Pul city, the government buildings and all the installations there."

The Taliban had the compound "surrounded", said Mohammad Hussein Mujahidzada, a member of the provincial council.

   **- Perennial target -**

Kunduz is the most significant Taliban gain since the insurgents launched an offensive in May as foreign forces began the final stages of their withdrawal.

It has been a perennial target for the Taliban, who briefly overran the city in 2015 and again in 2016 but never managed to hold it for long.

The ministry of defence said government forces were fighting to retake key installations.

"The commando forces have launched a clearing operation. Some areas, including the national radio and TV buildings, have been cleared of the terrorist Taliban," it said.

Kabul's ability to hold the north may prove crucial to the government's long-term survival.

Northern Afghanistan has long been considered an anti-Taliban stronghold that saw some of the stiffest resistance to militant rule in the 1990s.

The region remains home to several militias and is also a fertile recruiting ground for the country's armed forces.

"The capture of Kunduz is quite significant because it will free up a large number of Taliban forces who might then be mobilised in other parts of the north," said Ibraheem Thurial, a consultant for International Crisis Group.

Vivid footage of the fighting was posted on social media over the weekend, including what appeared to be large numbers of prisoners being freed from jails in captured cities.

The Taliban frequently target prisons to release incarcerated fighters to replenish their ranks.

On Friday, the insurgents seized their first provincial capital, Zaranj in southwestern Nimroz on the border with Iran, and followed it up by taking Sheberghan in northern Jawzjan province the next day.

   **- US air strikes -**

Fighting was also reported on the outskirts of Herat in the west, and Lashkar Gah and Kandahar in the south.

The pace of Taliban advances has caught government forces flatfooted, but they won some respite late Saturday after US warplanes bombed Taliban positions in Sheberghan.

"US forces have conducted several air strikes in defence of our Afghan partners in recent days," Major Nicole Ferrara, a Central Command spokesperson, told AFP in Washington.

Sheberghan is the stronghold of notorious Afghan warlord Abdul Rashid Dostum, whose militiamen and government forces were reportedly retreating east to Mazar-i-Sharif in Balkh province.

Dostum has overseen one of the largest militias in the north and garnered a fearsome reputation fighting the Taliban in the 1990s -- along with accusations his forces massacred thousands of insurgent prisoners of war.

A retreat of his fighters dents the government's recent hopes that militias could help the overstretched military.

The government has said little about the fall of the provincial capitals, other than vowing to retake them.

That has been a familiar response to most Taliban gains in recent weeks, although government forces have largely failed to make good on promises to retake dozens of districts and border posts.