Hundreds of mercenaries from Russia's shadowy Wagner Group are operating in Libya, says a released UN report.
They are supporting Renegade General Khalifa Haftar's battle against the Tripoli government, the document says.
Since 2014, Libya has been divided into areas controlled by the sides.
Wagner is believed to be involved in conflicts in Ukraine, Syria and elsewhere. The Russian government denies any state involvement in the militant group.
What does the UN report say?
The report was compiled by independent monitors for the UN Libya sanctions committee. It hasn't been released yet, but it has been seen by news outlets.
It is the first time that the UN confirms reports of Wagner's involvement in Libya, putting the number of agents between 800 and 1,000.
The 57-page report says that Wagner agents have been in Libya since October 2018, "providing technical support for the repair of military vehicles and participating in combat operations".
Its members have served as artillery and air observers, in addition to "providing expertise in electronic countermeasures and standing out as teams of snipers".
The staff was mainly Russian, but there were also citizens of Belarus, Moldova, Serbia and Ukraine, according to the report.
Earlier this year, Russian President Vladimir Putin said that no Libyan citizens did not represent Moscow, nor were they paid by the state.
The UN report also says that Syrian mercenaries are fighting alongside General Haftar.
What is the background to the conflict in Libya?
Libya has been devastated by the war since the 2011 uprising, which toppled Muammar Gaddafi.
There is an internationally recognized national agreement government based in the capital, Tripoli. It is supported by Qatar and Turkey. Pro-Turkish Syrians are fighting for the GNA, which mainly controls areas in the northwest of the country.
In opposition to this is the Libyan National Army (LNA), loyal to Gen Haftar, which is in eastern Libya, but controls many other areas. It is supported by Egypt, Russia and the United Arab Emirates.
General Haftar launched an offensive against the capital in April 2019, but was unable to take it.
A truce brokered by Turkey and Russia in January was repeatedly violated.
What is known about Wagner?
It was reported that it was founded by Dmitry Utkin, who is believed to be a former member of a Russian military intelligence special forces brigade, the GRU. The media reported that Dmitry Utkin's call was Wagner, due to his admiration for the Nazi regime, which appropriated the composer's work to support his view of Germany.
Wagner made his presence felt for the first time in Military acquisition of Russia from the Crimean peninsula of Ukraine in March 2014 is at the pro-Russian insurgency that broke out in the Ukrainian regions of Donetsk and Luhansk the following month.
How the group is managed and financed is open to speculation. Some media sources suggest that the GRU secretly supervises him.
Investigations by Western journalists linked the wealthy Russian businessman nicknamed "chef de Putin" – Yevgeny Prigozhin – to the group. Prigozhin, part of Putin's elite circle, denied.
Analysts say that because the Russian government does not officially recognize the existence of the mercenaries, it can deny or minimize Russian casualties while maintaining a military presence.
Wagner has been reported to be operating in several conflict-affected nations.
A Russian BBC investigation in 2018 highlighted the death of Russian mercenaries fighting in Syria. About 2,500 members were serving there, with officers earning up to 300,000 rubles (£ 3,800; $ 5,300) a month.
Wagner's activities have also been reported in Madagascar and the Central African Republic (CAR). Three Russian journalists investigating Wagner's involvement were shot dead in the CAR in 2018.
What's new in the Libyan conflict?
The UN mission in Libya on Wednesday urged the two warring sides to resume negotiations on a lasting truce. They have been suspended since February.
General Haftar, whose forces have suffered setbacks in recent weeks, Wednesday called for a unilateral ceasefire for the month of Ramadan, but the GNA rejected it, saying it needed international safeguards.
The battle for the capital left hundreds dead, with more than 200,000 people displaced.
A GNA minister also accused General Haftar's forces, and Wagner in particular, of carrying out a chemical attack in the Salah al-Din area, south of Tripoli.
Interior Minister Fathi Bashaga said that GNA forces were "exposed to nervous gases, paralyzed and then cut off". There was no independent confirmation.
There were also reports of a loophole between Wagner and General Haftar about unpaid fees.