Afghan army troops have captured the Taleban-held town of Musa Qala without meeting resistance, the Nato-led force in Afghanistan has said.
The Taleban took control of the town in February
The insurgents have pulled out of what was the only major Afghan centre in Taleban hands, reportedly melting away into the mountains.
Afghan, US and UK troops have been fighting Taleban there since Friday.
The Taleban took over Musa Qala in February, despite a deal struck with tribal elders when UK troops withdrew.
It has since become the main centre of drugs trading in Afghanistan, the BBC's David Loyn in Kabul says.
The Taleban withdrew after telling local elders that they would not fight street by street after heavy aerial bombardment during the night.
According to our correspondent they will easily disappear into the mountains to the north of the town, dressed as they are in the same way as the local residents, with the distinctive black turban worn in the south of Afghanistan.
ASSAULT ON MUSA QALA
7 December: Major offensive begins, led by Afghan forces. US soldiers dropped by helicopter to carry out overnight assault
8 December: Twelve insurgents and two children reported dead after attack on town. One British soldier killed. British and Afghan troops take positions to south, west and east of town
9 December: Two men said to be senior Taleban leaders captured. One Nato soldier killed as town surrounded
10 December: Musa Qala re-taken by Afghan forces
However, they are expected to regroup and try to stage a counter-attack, our correspondent says, so the task for Nato and Afghan forces now is to dig in and fortify their positions.
The British are planning to set up a small base in Musa Qala, but the defence of the town will be led by Afghan forces.
As the assault was taking place, UK Prime Minister Gordon Brown was elsewhere in Helmand province, visiting troops in Camp Bastion, the largest British camp in the restive region.
Mr Brown made reference to the fighting in Musa Qala as he addressed troops there.
"I know this weekend in Musa Qala some of you here have been doing a very important job in clearing the Taleban from that area," Mr Brown said.
"I believe if we can succeed there, as we will, and if we can work with the Afghan forces, then we can move forward events in Afghanistan in favour of a more peaceful future for this country."
The assault is the first major operation where the new Afghan army is playing a leading role.
Mr Brown, at a later news conference with Afghan President Hamid Karzai in Kabul said the operation in Musa Qala was a prime example of Nato and Afghan troops working successfully together.
Twelve Taleban fighters and two children are reported to have been killed in the four days of fighting since Friday and a UK soldier died when an explosion hit his vehicle.
A second Nato soldier died in the area on Sunday.
Two senior Taleban leaders are reported to have been captured in the fighting.
Heavy rain fell on the battlefield overnight, making conditions difficult for vehicles as sand turns to mud.
It is also another hardship for thousands of residents of Musa Qala who fled north across the desert to the mountains when the fighting began.
On Sunday, the Taleban mounted attacks in three other towns in Helmand in an apparent diversionary tactic.