United States military officials say thousands of Nato-led troops in Afghanistan have launched the biggest offensive since the overthrow of the Taliban in 2001.
The operation, in the Marjah area, involves American, British and Afghan forces, the BBC reports. Afghan officials say five militants have been killed.
Nato says Marjah is home to the biggest community under insurgent control in southern Afghanistan.
Helicopter-borne US marines and Afghan troops are attacking Marjah in Nad Ali district in a bid to re-establish government control.
Many residents fled the area ahead of the offensive after Nato distributed leaflets warning of the planned attacks.
Villagers say the leaflets warned Taliban fighters to leave the area or be killed and the BBC reports the evidence suggests many did so.
The details of Operation Moshtarak were reportedly deliberately leaked to the media in the expectation that the insurgents would leave.
Correspondents say US and Afghan troops have moved much faster than expected, encountering little or no resistance.
On Friday, British forces began a "softening up" process, taking part in a Nato ground and air offensive in Helmand province.