Russian citizens working for the private Wagner Group military contractor have carried out clandestine missions on the Kremlin’s behalf in eastern Ukraine and Syria, people with ties to the group have alleged. Dozens of Wagner fighters were reportedly killed by U.S. airstrikes and artillery attacks in Syria earlier this year. Mercenaries are banned under Russian law and the Kremlin has regularly denied their presence in war-torn Syria.

Six Russian mercenaries were reportedly killed in an explosion at a military base in Syria, a Russian investigative newspaper reported Monday.

Six Wagner fighters were killed in the blast at a Syrian army base they were reinforcing, Russia’s investigative newspaper Novaya Gazeta reported Monday, citing an unnamed source in the country.

A total of 11 people were reportedly killed in Sunday’s explosion at a former criminal police building on a highway connecting the Syrian capital of Damascus with the eastern city of Deir Ezzor.

Novaya Gazeta did not indicate who may have been behind the explosion.

Russia’s Foreign Ministry has not yet commented on the alleged casualties.

Reuters contributed reporting to this article.