U.S. concerns about the arrival of Russian fighter jets in Libya have been compounded by evidence those planes are being flown by inexperienced pilots, military officials said Thursday, according to Business Insider.

Reuters reported in early May that the UN found up to 1,200 personnel from Russian private military contractor Wagner Group were in Libya to support warlord Khalifa Haftar in his fight against the country's internationally recognized government. Africom said Thursday about 2,000 Wagner Group personnel were estimated to be in Libya.

The aircraft were flown from Russia to Syria, where their Russian markings were painted over, and then to Libya. That violated a UN arms embargo and indicated that Russia was "clearly trying to tip the scales in its favor in Libya," U.S. Army Gen. Stephen Townsend, head of Africom, said at the time.

On Thursday, Africom warned of what it said was a lack of skill among the aircraft's pilots.

Russian aircraft being used to support private military companies sponsored by the Russian government. The Spoon Rest is a Russian-made, mobile early warning radar system, designed to provide tactical support to military activities. Photo release June 18, 2020. U.S. Africa Command

"There is concern these Russian aircraft are being flown by inexperienced, non-state PMC mercenaries who will not adhere to international law" and aren't bound by the laws of armed conflict, said U.S. Marine Corps Brig. Gen. Bradford Gering, Africom's director of operations. "If this is true and bombing occurs, innocent Libyan lives are at risk."

At a Defense Writers Group event on Thursday, Gen. Jeffrey Harrigian, commander of U.S. Air Forces in Europe and Africa, told reporters that the Russian aircraft and their pilots have largely been working on "pilot proficiency."

"Primarily what we've seen then as far as activity is, in essence, what I would quantify as sorties where they're working on their basic flying skills," Harrigian said,

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Africom said a MiG-29 was photographed operating near the city of Sirte in northern Libya, but Harrigan said the U.S. was unable to confirm Wagner Group's involvement in strikes reported in that area.

Harrigan described the pilots as "guys that may be retired" and "guys that they're finding out there who have flown these types of aircraft."

"Naturally, from a professional airman's perspective, that's going to raise some concern. It ought to raise concern in everybody's mind in terms of what's their ability to put weapons on the appropriate targets," Harrigan added.

"It's a small, constrained environment to operate in, where it takes precision weapons," Harrigan said. "So when you haven't had potentially the opportunity to operate with precision weapons in a manner that's going to require tactics, techniques, and procedures — that are a skill set that we trained significantly to ensure that we put weapons where we need to — that's an area that we have concern."

U.S. military officials have characterized the presence of Russian weapons in Libya as a potential threat to NATO.