Between 10 and 35 Russian mercenaries may have died while fighting in Libya’s civil war that has plunged the oil-producing nation into crisis, the Meduza news website has reported, citing unnamed colleagues and veterans.

Forces loyal to eastern commander Khalifa Haftar in April launched an offensive to take Tripoli, held by Libya’s internationally recognized government, upending UN-led plans to broker a political settlement. Russia is reportedly providing unofficial assistance to Haftar with a private army linked to President Vladimir Putin, and an unnamed Russian mercenary commander told Bloomberg that contractors had been killed in action last month.

“They were hit in an airstrike on the frontline,” an unnamed Ukrainian war veteran close to two of the Russian mercenaries killed in Libya told Meduza. The outlet identified these two mercenaries as Artyom Nevyantsev and Ignat Borichev.

One source said the mercenaries who came under attack last month were meant to lead an assault on Tripoli. Another speculated that Libyan government forces had intentionally struck the Russian mercenaries.

Bloomberg reported that more than 100 mercenaries from the Wagner group arrived at a forward base in Libya in the first week of September to support Haftar’s assault. Wagner is headed by Yevgeny Prigozhin, a businessman known as “Putin’s chef” for his Kremlin catering contracts.

Various sources told Meduza that the casualties came mostly from central Russia’s Sverdlovsk region and that their bodies will not be repatriated until late October. At least two combatants were reported to have been transported for burial back home.

Libya’s conflict has increasingly become a proxy war between foreign powers, which have been backing various armed groups since the 2011 uprising against Muammar Gaddafi. Haftar’s campaign has displaced more than 120,000 people and derailed years of UN peace efforts but gone nowhere as his forces have been unable to breach Tripoli’s southern suburbs.