Not a very conclusive or persuasive argument since every Ukraine male has to do compulsory military service (either one year as a soldier or two years as an officer).
I am not claiming that there are no Russian soldiers in Ukraine, but I take a very jaundiced view of the claims made by UK and US politicians about Russian soldiers in Ukraine as they have been caught lying too often before (Kuwaiti incubator babies, Jessica Lynch comedy stories, MI6 dodgy dossier).
They have AK-103's which arn't even available in Ukraine, beside the point that the soldiers identified themselves as Russian
Armed Russian Soldiers in Slovyans?k | Ukraine Investigation
The 'claim' that the soldiers are Russian doesn't come from UK/US politicians, it comes from Ukrainians. The mayor of Slovyans’k described how they (100men) arrived from outside the town and were not welcomed bu most of the townsfolk.