Jordan, Turkey and the United Arab Emirates have regularly violated the UN arms embargo imposed on Libya since 2011, according to a confidential report by UN experts seen Thursday by AFP.
The three countries "routinely and sometimes blatantly supplied weapons with little effort to disguise the source," a summary of a year-long study by UN experts said.
According to diplomats, Jordan was accused of having trained troops of Khalifa Haftar, a military strongman in eastern Libya who launched an offensive in April in a bid to seize Tripoli.
The United Arab Emirates, another Haftar backer, is suspected of using attack aircraft on behalf of his forces.
Turkey, which openly supports the government of Prime Minister Fayez al-Sarraj, supplied his forces with military material ranging from armored vehicles to drones, according to the sources.
"Both parties to the conflict received weapons and military equipment, technical support... in non-compliance with the sanctions measures related to arms," said the experts' report, delivered to members of the UN Security Council October 29.
The 85-page document and a more than 300-page annex includes pictures, maps and copies of ship manifests of cargos delivered to Libya by sea.
The report is expected to be the subject of debate by the Security Council's 15 members at the end of the month in the sanctions committee responsible for Libya. It is then expected to be approved for public release, probably in December.
"The panel identified multiple acts that posed a threat to the security, peace and stability of Libya," the experts said.
Since Haftar's offensive in April, a "new phase of instability, combined with the interests of several states and non state actors in the outcome, amplified the existing proxy conflict that took shape post-2011," they said.
"Military operations have been dominated by the use of precision-guided munitions from unmanned combat aerial vehicles, which to a degree has limited the collateral damage normally expected from such a conflict," they said.
The use of drones "has been massive by both sides," a diplomat said, confirming accusations leveled previously by the UN envoy for Libya, Ghassan Salame.
Another diplomat, also speaking on condition of anonymity, said the report does not mention the presence of Russian mercenaries in Libya.
Several hundred have been deployed in Libyan territory in recent months, taking part in combat in support of Haftar's forces, US media reported earlier this week.
The conflict in Syria has been fueled by arms supplied to both sides by foreign powers in violation of a UN arms embargo, a US study concludes