In a separate incident, a policeman killed his nine fellows in their sleep in the northern city of Kunduz.
Taliban fighters have captured the key Sangin district in Afghanistan's southern Helmand province on Thursday, underlining its growing strength in the country's south.
The Taliban effectively control or contest 10 of the 14 districts in Helmand, the deadliest province for British and US troops over the past decade.
Taliban spokesman Qari Yousuf Ahmadi said the group captured a military base and police headquarters following the withdrawal of Afghan troops and police from the Sangin district.
Helmand governorate spokesman Omar Zwak said security forces had conducted a tactical withdrawal some three kilometres from the district centre to avoid civilian casualties.
Insider attack in Kunduz
In a separate attack, a policeman linked to the Taliban killed nine of his fellows as they were sleeping in the northern Kunduz province, a local police chief said.
Taliban spokesman Zabiullah Mujahed claimed responsibility for the attack, but denied that a policeman had been involved or that the Taliban had burned the bodies of the policemen.
Insider attacks have been a major problem during the more than 15-year-long war.
Last week, three US soldiers were wounded when an Afghan soldier opened fire in Helmand, in the first known insider attack on international forces this year.