https://en.wikipedia.org/wiki/Russian_separatist_forces_in_Donbas said:
it’s widely believed that the separatists are supported by the Russian Armed Forces. Ukraine, the United States, and some analysts consider 1st and 2nd Army Corps to be Russian formations under the command of the 8th Combined Arms Army, which was formed up in 2017, in Novocherkassk, Rostov oblast. Although the Russian government often denies direct involvement, stating that their soldiers were there voluntarily and not under orders, some of them were captured with documents that said otherwise. The separatists have admitted receiving supplies from Russia and being trained there. BBC reported that separatist ranks are composed of thousands of Russian citizens.[18][19] Registered Cossacks of the Russian Federation have been reported to also be supporting the separatists. DPR head Alexander Zakharchenko claimed in August 2014 that there are around 3,000 to 4,000 Russian volunteers fighting for the militia, which includes current and many retired Russian Army servicemen.
Since September 2015, the separatist units, at the battalion level and up, are acting under direct command of Russian Army officers, with former local commanders sometimes serving as their deputies.