The MiG-21 crash-landed in a residential area south of Tobruk, where a rival to Libya's UN-backed unity government is based, 1,200 kilometres east of Tripoli.
A source at a Tobruk military base, who declined to be named, said the jet had carried out bombing raids on jihadist positions in Derna, 100 kilometres west of Tobruk, before crashing.
The pilot, a member of forces loyal to Libyan military strongman Khalifa Haftar, was killed but the source was unable to say if the incident caused any other casualties.
Libya has been riven by fighting since the NATO-backed ouster of longtime dictator Moamer Kadhafi, with rival forces vying for territory and the country's vast oil wealth.
More From This Section
The chaos has allowed jihadist fighters including the Islamic State group to gain a toehold on pockets of the Mediterranean coast.
Haftar supports the Tobruk-based authorities and opposes the UN-backed Government of National Accord, which controls Tripoli but has struggled to assert itself outside the capital.
Haftar's forces have battled jihadists in second city Benghazi for more than two years. They have lost several aircraft in recent months.