The US military has accused Russia of sending Libyan warplanes in support of Russian alliances with General Khalifa Haftar.
The head of the US Air Force in Africa, General Stephen Townsend, said earlier that Russian warplanes had set up camp in Syria before they arrived in Libya.
He did not say when the planes arrived in the country.
The allegations come a day after the UN-backed Libyan government said hundreds of Russian fighters had been evacuated from the battlefield south of Tripoli.
Russia has repeatedly denied the allegations.
