While the ongoing Syrian ceasefire has stopped the fighting between the military and most rebel factions, the explicit exclusion of ISIS from the deal has allowed everyone to target ISIS territory with impunity. For the Syrian government, that attention has centered on Palmyra.
An ancient oasis town, Palmyra is itself largely unpopulated, but adjacent to a key, similarly ISIS-held city called Tadmur. The Syrian state media reports that the most recent offensive aimed at retaking the area has killed or wounded scores of ISIS fighters.
Though there had been no reports of Russian troops on the ground in that battle, ISIS claimed to have killed five Russian soldiers, along with six Syrian soldiers, and several Hezbollah, in the fighting. The Russians may well have been the “mercenaries” the Assad government has been recruiting, retired Russian soldiers with no direct Russian government ties.