It was not immediately clear why the aircraft came down, AFP reported.
The victims included General Idris Yunis al-Dressi, chief of staff for Aguila Saleh, speaker of the internationally recognized parliament based in Tobruk in eastern Libya.
His son Ahmed Idris al-Dressi, who worked in the parliament, was also killed along with two air force colonels and one lieutenant.
The four officers belonged to the forces under the command of military strongman Khalifa Haftar, who has refused to get behind a UN-backed unity government and supports the parallel authority based in eastern Libya near the border with Egypt aligned with the Tobruk parliament.
According to a statement posted on parliament's website, the helicopter was returning from an official visit to Ras Lanuf, a key oil export terminal.
Last week, forces led by Haftar seized Ras Lanuf and three other ports in the so-called "oil crescent".
They later handed them over to the National Oil Corporation.
On Sunday fighters loyal to the unity government launched an attack aimed at retaking two of the ports but were repelled.