Private military contractor Wagner Group is conducting the hiring with Russian army supervision, opposition sources say.
The European Union’s top diplomat has urged all parties in the conflict in Libya to immediately stop all military operations and engage constructively in peace negotiations.
As Turkish drones helped drive eastern Libyan forces back from Tripoli this month, Russia was said to be reinforcing the forces with warplanes, raising the stakes in a civil war that has partitioned the north African country.
Recent weeks have marked a turning point in a complex conflict between two uneasy coalitions that are each backed by an array of foreign states, whose competing regional agendas make them unwilling to countenance defeat.
In a joint statement with the foreign ministers of Germany, France and Italy issued on Tuesday, the EU’s top diplomat, Josep Borrell, called on the conflict parties in Libya to swiftly agree on a ceasefire and withdraw all foreign forces, mercenaries and military equipment.
The joint statement followed increased diplomatic efforts by Germany to push for a political solution to the Libya crisis.
Chancellor Angela Merkel earlier on Tuesday expressed her concern in a telephone call with Russian President Vladimir Putin over the recent escalation of fighting in Libya.
On Monday, Merkel discussed the situation in the country with Egyptian President Abdel Fattah al-Sisi.
Merkel told el-Sisi that United Nations-backed negotiations must remain the key aim of the peace process where the internationally recognised Government of National Accord (GNA) is fighting General Khalifa Haftar’s self-styled Libyan National Army in the east.
On Saturday, el-Sisi proposed a new ceasefire after the Turkish-backed GNA won a series of rapid victories over Haftar’s forces, dashing Haftar’s bid to unite the country by force with help from Egypt, the United Arab Emirates and Russia.
Buoyed by their recent military victories in western Libya, GNA forces have launched an offensive to capture Sirte, with fighting so far concentrated on the city’s western and southern outskirts.
Known for being the birthplace of former longtime ruler Muammar Gaddafi, Sirte holds significant symbolic value as it is located roughly halfway between Tripoli and Haftar’s eastern bastion of Benghazi.