A spokesman for the self-proclaimed Libyan National Army (LNA) said its fighters had advanced in "several regions in the south" from an airbase some 650 kilometres (400 miles) from the capital Tripoli.
The aim is to "assure security for inhabitants in the south-west from terrorists, be they the Islamic State group or Al-Qaeda, as well as criminal gangs," spokesman Ahmed al-Mesmari said.
The LNA said it was also looking to secure petroleum facilities and tackle flows of clandestine migrants heading northwards to the Mediterranean coast.
It called on armed groups in the target area, mainly made up of tribal fighters, to withdraw from military and civilian installations.
Military sources told AFP that numerous LNA units had taken up positions in recent days around the region's main city of Sabha.
Libya has been torn between rival administrations and a myriad of militias since the NATO-backed overthrow and killing of dictator Moamer Kadhafi in 2011.
Haftar supports an administration in the east of the country that is opposed to the internationally backed Government of National Accord in Tripoli.
The chaos has seen jihadists and people traffickers gain a foothold in the south of the country.
IS has carried out repeated attacks across the country, targeting both Haftar's forces and the rival Tripoli-based authorities.