No evidence given to support this claim. The existence of paramilitary fighters or mercenaries from NATO or US-friendly countries is a recurrent narrative in Russian disinformation since at least 2014, and it prompted a diplomatic protest from Poland back then.

The allegation has been repeatedly debunked in those years, but resurfaces again and again since it is useful to try to discredit the Ukrainian army and the countries that support Kiev efforts for stabilization. Many false incidents have been attributed this ‘ghost army’, from rapes to vendettas against Ukrainian soldiers due to low morale.

See here for other disinformation cases about alleged presence of foreign mercenaries in Eastern Ukraine, from Baltic snipers to snipers from Georgia, female snipers from Poland, and even a whole NATO brigade.