Itamilradar, a website that tracks military flights, said it had tracked several flights operated by Russian Air Force aircraft to eastern Libya.
The website added that up to eight flights per day have been registered since February, all operated via Latakia, Syria.
The aircraft used were Tupolev Tu-134, Tu-154 and Ilyushin Il-76.
The website also tracked a Syrian Air Force Ilyushin Il-76 flying from Syria to Libya on Tuesday.
Both Russia and Syria are strong backers of warlord Khalifa Haftar.