According to the Criminal Code of the Republic, mercenary activity is punishable by imprisonment for 3 to 7 years. Forcible seizure of power in the DPR is punished by imprisonment from 12 to 20 years, and in wartime they can be replaced by the death penalty. Mercenary activity is recognized as a crime by international law and the Geneva Convention does not apply to mercenaries.
[southfront.org]
Mercenary activity when the nation is not at war with nor sanctioning the activity is one definition of treason. "Making war on a nation that is not at war with the United States."