A force of several dozen armed private security contractors from Russia operated until last month in a part of Libya that is under the control of regional leader Khalifa Haftar, Reuters reported the head of the firm that hired the contractors as saying.
It is the clearest signal to date that Moscow is prepared to back up its public diplomatic support for Haftar – even at the risk of alarming Western governments already irked at Russia's intervention in Syria to prop up dictator President Bashar Al-Assad.
Haftar is opposed to a UN-backed government which Western states see as the best chance of restoring stability in Libya. But some Russian policymakers see the Libyan as a strongman who can end the six years of anarchy that followed the ousting of veteran tyrant Muammar Gaddafi.
The presence of the military contractors was, according to the head of the firm, a commercial arrangement. It is unlikely though to have been possible without Moscow's approval, according to people who work in the industry in Russia.
Oleg Krinitsyn, owner of private Russian firm RSB-group, said he sent the contractors to eastern Libya last year and they were pulled out in February having completed their mission.
In an interview with Reuters, he said their task was to remove mines from an industrial facility near the eastern Libyan city of Benghazi, in an area that Haftar's forces had captured from rebels.
He declined to say who hired his firm to provide the contractors, where they were operating or what the industrial facility was. He did not say if the operation had been approved by the UN-backed government, which most states view as the sovereign ruler of Libya.
Asked whether the mission had official blessing from Moscow, Krinitsyn said his firm did not work with the Russian defence ministry, but was "consulting" with the Russian foreign ministry.
The contractors did not take part in combat, Krinitsyn said, but they were armed with weapons they obtained in Libya. He declined to specify what type of weapons. A UN arms embargo prohibits the import of weapons to Libya unless it is under the control of the UN-backed government.
Krinitsyn said his contractors were ready to strike back in case of an attack.
"If we're under assault we enter the battle, of course, to protect our lives and the lives of our clients," Krinitsyn said. "According to military science, a counterattack must follow an attack. That means we would have to destroy the enemy."
Military and government officials in eastern Libya said they were not aware of the presence of the contractors, while Haftar remained tight-lipped.
Haftar has been seeking outside help to consolidate his control over parts of Libya. Russia has shown a willingness to engage with him that contrasts with the more cautious approach of Western governments.
Haftar visited Moscow in November last year and met Russian Foreign Minister Sergei Lavrov. In December, Haftar went on board a Russian aircraft carrier off the Libyan coast and spoke with the Russian defence minister via videolink. In recent weeks, Russia has taken in 100 of Haftar's wounded fighters for medical treatment.