Taliban militants were spotted on Monday near the centre of Lashkar Gah, a city that once housed the headquarters of British Army operations in the country between 2006 and 2014.
The Afghan government has stationed commandos in the city, capital of the fertile Helmand province in the country's south and home to several large military bases.
Helmand is one of several Afghan provinces where government forces have been all but routed since a Taliban offensive began in the spring, although government control has long been slipping in many rural areas.
Last month, the Taliban took control of the town of Spin Boldak on the border with Pakistan, leading to what the US embassy in Kabul called a massacre of civilians.
In 2017, the Taliban took control of Helmand's strategically important Sangin district, cutting a critical road to Lashkar Gah, an area that was once the site of heavy fighting between the Taliban and international coalition forces.
Despite the precipitous withdrawal of international support, Afghanistan's army has managed to hold on to important urban areas.
That control is now in doubt, but on Monday, ground and air forces managed to repel an assault on Lashkar Gah, the Afghan military said.
Services in the city have all but collapsed and in the past several months the food supply in the province has dwindled.
"There is fighting, power cuts [and] sick people in hospital; the telecommunication networks are down. There are no medicines and pharmacies are closed", a resident told AFP.
Afghan army near collapse
"The Afghan army is facing a major morale crisis right now," said Faran Jeffery, a security analyst who focuses on terrorism and insurgency in Afghanistan and Pakistan.
"The troops are very demoralised since the US withdrawal announcement in May. Their reliance on local militias has also backfired on several occasions where the Taliban only succeeded because the militiamen switched sides and helped defeat the army."
While the Taliban face internal divisions based on different local agendas, Afghanistan's security forces are highly fragmented on tribal and ethnic lines.
Nonetheless, the militant group has proven adept at organising forces at the local level.
"Militia defections are particularly helpful for the Taliban because they get all the insider information from these defectors, which makes it very easy for them to launch precise attacks," Mr Jeffery said.
Lashkar Gah is one of three Afghan cities close to being overrun by the militants.
In the capital of neighbouring Kandahar province, often regarded as the city where the Taliban first took root in the country, at least 22,000 families have fled fighting.
Nationally, the number of displaced people in Afghanistan has surged.
According to the UN Office for the Co-ordination of Humanitarian Affairs (OCHA), the number of Afghans fleeing their homes since July last year has increased by 101 per cent.
In the western part of the country, Taliban militants were pushing deeper into the city of Herat on Monday, where the government has also stationed commandos.
But the government strategy of using its best troops in crisis-hit areas could have serious limitations, Mr Jeffery said.
"The Afghan government has been relying mainly on Special Operations Forces [SOF] to hold cities but that has a price," he said.
"It means that these SOF troops are very much overstretched and while they're trying to defend one or two cities, Taliban launch attacks in other areas and quickly end up capturing new areas. Sooner or later, the SOF troops will be too exhausted."
The country's fledgling air force has been conducting strikes against groups of Taliban fighters, with the US Air Force joining, although in a limited capacity.
That has led to a surge in civilian casualties in towns such as Lashkar Gah, according to Doctors Without Borders.
Afghan President Ashraf Ghani blamed the country's deteriorating security situation on Washington's decision to rapidly withdraw.
"The reason for our current situation is that the decision was taken abruptly," he told Parliament on Monday.
Mr Ghani said he had warned Washington that the withdrawal would have consequences.
A turning point in Helmand
The capture of Lashkar Gah would be hugely symbolic for the Taliban. Helmand province saw some of the bloodiest fighting in the conflict between international forces and the Taliban after 2001, with British forces facing almost siege-like conditions in remote outposts in 2006.
Later, when US president Barack Obama decided to "surge" US forces into the country in a final push to crush the Taliban, Helmand was once again in the spotlight.
US Marines fought a vicious campaign to gain control of districts including Garmsir in the north of the province and Marjah in the centre, through 2009-2011.
But the stalemate could not be broken and the province remains an important centre for poppy cultivation for the production of opium. Between 2019 and 2020, the UN Office on Drugs and Crime reported a 37 per cent increase in cultivation of the plant.
It remains to be seen whether Afghan forces will have any more success than foreign troops.
But Mr Jeffery says that the loss of one of the three cities now besieged – Kandahar, Lashkar Gah and Herat – is not a given.
"I still think that it won't be very easy for the Taliban to capture key cities," he said.
"They are very close to capturing Lashkar Gah for instance, but even there, their advances can be rolled back in coming hours or days."
Some cities might hold due to specific local factors, such as the presence of experienced commanders.
"Herat city will hold as long as Ismail Khan [a commander with notable combat experience] is fighting, and his militiamen don't surrender," Mr Jeffery said.
"Ismail Khan, being a war veteran that he is, knows very well that SOF troops alone cannot be expected to hold the city."