“We are concerned about reports that illegitimate authorities in eastern Ukraine will bring charges against residents of the UK, Sweden and Croatia,” he wrote on Twitter.
Blinken added that Russia and its allies must respect international humanitarian law, including the rights and protection afforded to prisoners of war.
Earlier, the Prosecutor General's Office of the Donetsk People's Republic sent to court a criminal case against mercenaries from Croatia, Sweden and the UK.
The department emphasized that in wartime conditions, mercenaries face life imprisonment or the death penalty.