Fighting has been intense in the city of Lashkar Gah since Friday.
Afghan and US air strikes have reportedly targeted Taliban positions and government forces say they have killed dozens of militants.
One resident claims Taliban fighters have surrounded the police headquarters and they have been searching houses and throwing people out of their homes.
There are fears the city is at serious risk of falling to the Taliban and, if that happens, experts believe several other provinces in the region would also be lost.
Lashkar Gah is just one of three cities under attack. Militants are also targeting Kandahar in the south and Herat to the west.
Flights at Kandahar airport were suspended after it was hit by Taliban rockets in the early hours of Sunday.
Fighting has surged across the country in recent months amid the withdrawal of foreign forces.
A small number of British military personnel will temporarily remain in Afghanistan to support the transition to a new phase of UK support for the country, the Ministry of Defence (MOD) said.
Cover image: Firearm damage seen in Lashkar Gah seen amid sustained attack from the Taliban.