The official said an attacker drove an explosives-laden car into the checkpoint, located about 60 miles west of Benghazi on the road to Ajdabiya.
Libya has plunged into chaos since the fall of dictator Moamer Kadhafi in October 2011 after a NATO-backed revolt, with rival governments and militias battling to control the oil-rich North African country.
There was no immediate claim of responsibility for Tuesday's attack but government forces have been battling Islamist militia for control of Benghazi for months.
Libya's internationally recognised government, which has lost control of much of the country, has pleaded for international help in combating the militias.