The warplanes left Russia and first stopped in Syria, where they "were repainted to camouflage their Russian origin" before arriving in Libya, said the US military command for Africa (Africom) in a statement.

Stuttgart-based Africom did not specify when the jets flew in, saying only that it was "recently".

But it appeared to be the latest violation of the nine-year-old UN embargo on sending arms into Libya.

In Washington, Pentagon spokesman Jonathan Hoffman said that the Russians had sent a mix of "approximately 14 military fighters."

Africom posted pictures of the Russian aircraft on its Twitter feed, including one that it said showed several MiG-29 Fulcrum jets and Su-35 Flankers parked at an airbase.

- Russian mercenaries evacuated -

The claim comes a day after Libya's UN-recognised government said hundreds of Russian mercenaries backing rival military commander Haftar had been evacuated from combat zones south of the capital Tripoli.

The alleged retreat follows a series of setbacks for Haftar's years-long offensive to seize the capital from the Tripoli-based Government of National Accord (GNA).

The Kremlin has always denied involvement in the conflict.

When contacted by AFP, the Russian defence ministry said it would "not comment right now" on the Africom statement.

But Andrei Krasov, a member of the defence committee in Russia's lower house of parliament, dismissed the allegation as "fake" news.

Separately, Russian Foreign Minister Sergei Lavrov reaffirmed the need for an immediate ceasefire and "constructive dialogue" in Libya, according to his ministry, following talks with the speaker of Libya's parliament.

Hoffman said outsiders need to stop stoking violence in Libya.

"We have asked for all parties that are operating, that's the Russians and the Turks and others, to cease their behavior there. We want a secure and stable Libya."

- 'No denying it now' -

United Nations experts said in a report last month that the Wagner Group, a shadowy Russian paramilitary organisation seen as close to President Vladimir Putin, had sent fighters to back Haftar.

The report, submitted to the UN Security Council, estimated there were between 800 and 1,200 Wagner mercenaries in Libya.

"For too long, Russia has denied the full extent of its involvement in the ongoing Libyan conflict. Well, there is no denying it now," said US Army General Stephen Townsend in the Africom statement.

"Russia is clearly trying to tip the scales in its favour in Libya," Townsend said.

- Setbacks for Haftar -

Oil-rich Libya plunged into chaos after veteran dictator Moamer Kadhafi was ousted in a 2011 NATO-backed uprising.

Rival administrations and militias have been vying for power ever since.

The unrest worsened when Haftar -- who is also backed by Egypt, the United Arab Emirates and Saudi Arabia -- launched an offensive on Tripoli in April 2019.

The GNA, gets support from Turkey, whose drones and air defence systems helped deliver key victories in recent weeks.

In a major blow for Haftar, GNA forces last week captured Al-Watiya airbase, where Haftar had stationed aircraft for bombing runs.

Africom said Russia's actions risked prolonging the conflict and exacerbating "casualties and human suffering on both sides".

If confirmed, Russia's fighter jet deployment would constitute another violation of a much-abused 2011 UN arms embargo.

World leaders agreed in January to uphold the embargo and stop meddling in the conflict, but the UN has repeatedly warned that both sides have continued to receive arms and fighters.

Separately, the US Navy said that Russian SU-35 fighters flew dangerously close to a US P8-A surveillance aircraft over the eastern Mediterranean on Tuesday, the third such "unsafe and unprofessional" interception in the past two months.