In a strongly worded statement, the U.S. Africa Command said the jets were flown from an air base in Russia first to Syria, where they were “repainted to camouflage their Russian origin.” According to Libyan officials, diplomats and analysts, at least eight jets were then sent to eastern Libya, arriving last week at a base controlled by commander Khalifa Hifter, who launched an offensive on the capital Tripoli last year.

The aircraft, the statement said, are “likely to provide close air support and offensive fires” for Russian mercenaries working for the Wagner Group, a private army that experts have linked to Yevgeniy Prigozhin, a close ally of Russian President Vladimir Putin.

Since September, hundreds of Russian mercenaries have fought on Tripoli’s front lines alongside Hifter’s forces, who seek to topple the capital’s U.N.-backed government. But virtually all of the mercenaries left Tripoli over the weekend after Hifter’s forces suffered military reversals in western Libya.

“Russia is clearly trying to tip the scales in its favor in Libya,” U.S. Army Gen. Stephen Townsend, commander of U.S. Africa Command, said in the statement. “Just like I saw them doing in Syria, they are expanding their military footprint in Africa using government-supported mercenary groups like Wagner.”

He added: “For too long, Russia has denied the full extent of its involvement in the ongoing Libyan conflict. Well, there is no denying it now.”

Andrei Krasov, deputy head of the defense committee in the lower house of Russia’s parliament, denied that Russia has deployed military planes to Libya. “Another U.S.-style horror story. That’s a fake and misinformation presented in the spirit of the previous U.S. administration,” Krasov told the Interfax news agency. “Once again, the U.S. is trying to play the Russia card.”

He said the Russian position “is well known: We want the bloodshed to stop in Libya, and we call on all conflicting parties not to use force but to begin negotiations.”

The North African oil-producing nation is in the grips of its worst bloodshed since dictator Moammar Gaddafi was toppled and later killed during the 2011 Arab Spring revolts and NATO intervention. The war is being fueled by regional and European powers, especially Turkey, Russia, the United Arab Emirates, France and Egypt, which have backed the warring sides for a variety of interests, including lucrative oil and gas contracts, territory, and ideological and strategic goals.

Turkey and Russia, in particular, have deepened their military and political imprints on Libya in recent months, raising fears they could confront each other and plunge Libya into a Syria-like conflict. Turkish military support for forces aligned with the Tripoli government led to Hifter’s defeats in recent weeks.

“The deployment of military fighter aircraft by #Moscow at this juncture would yield political & economic dividends for Moscow in #Libya,” Emadeddin Badi, a Libya analyst with the Atlantic Council, said in a tweet. “It entrenches #Russia’s position as #Turkey’s opponent, increases its sway over [Hifter] specifically & the Eastern #Libya block more broadly.”

In November, Putin’s spokesman, Dmitry Peskov, declined to answer questions sent by The Washington Post about the mercenaries, replying that “the Kremlin does not have this information.” A spokesman for Prigozhin said the businessman “has nothing to do with the so-called ‘Wagner’ private military company” and declined to comment further.

The U.S. Africa Command said that “Russia has employed the state-sponsored Wagner Group in Libya to conceal its direct role and to afford Moscow plausible deniability of its malign actions.” It added that Moscow’s military actions have prolonged Libya’s war and worsened the number of casualties and human suffering.

In its statement, the U.S. Africa Command said that “Russia is not interested in what is best for the Libyan people.”

Townsend described the fighter jets as “fourth generation” aircraft that the Wagner mercenaries and the Libyan National Army, as Hifter’s forces call themselves, are not trained to fly.

“Neither the LNA nor private military companies can arm, operate and sustain these fighters without state support — support they are getting from Russia,” Townsend said. “The world heard Mr. [Hifter] declare he was about to unleash a new air campaign. That will be Russian mercenary pilots flying Russian-supplied aircraft to bomb Libyans.”

Moscow’s actions, the U.S. Africa Command added, could aggravate the regional instability that has driven hundreds of thousands of migrants to cross the Mediterranean Sea toward Europe.

Russia was also interested in using Libya as a base to achieve its own strategic goals, the statement said. That could include bringing “anti-access area denial” systems, including air defenses and offensive strike weaponry such as ballistic missiles, to prevent Russian foes from accessing Libya and parts of North Africa.

“If Russia seizes basing on Libya’s coast, the next logical step is they deploy permanent long-range anti-access area denial capabilities,” said Air Force Gen. Jeff Harrigian, commander of U.S. Air Forces in Europe-Air Forces Africa. “If that day comes, it will create very real security concerns on Europe’s southern flank.”

On Tuesday, Russian Foreign Minister Sergei Lavrov called for an immediate cease-fire in Libya and said attempts to resolve the Libyan crisis by force had no future, according to the Russian Foreign Ministry. Lavrov, speaking by phone to the speaker of the Libyan House of Representatives, Aguila Saleh, called for negotiations between Libyan parties to the conflict to end the hostilities.