The blast - which was not immediately claimed by any group - wounded two others at a checkpoint on the road leading to Tripoli, some 500 kilometres (310 miles) to the north, said commander Ahmed al-Mesmari.
Jihadists from the Islamic State group remain active in central and southern Libya despite being forced out of their northern stronghold Sirte in 2016.
Libya has been gripped by chaos since a NATO-backed uprising toppled and killed longtime dictator Moamer Kadhafi in 2011, with rival administrations and multiple militias vying for control of the oil-rich country.