The comments come after he recently called the Ukrainian leader a "dictator without elections" - apparently in response to Mr Zelenskyy saying his US counterpart was living in a "disinformation space" after Mr Trump claimed Ukraine had started the war.
Mr Kellogg struck a markedly different tone to President Trump when he called Mr Zelenskyy the "courageous leader of a nation at war".
Pic: Reuters] Mr Trump made the comments days before both leaders visit the White House for a meeting in which they must try to press Ukraine's case while keeping the US leader onside.
Sir Keir Starmer and Emmanuel Macron have not "done anything" to end the Ukraine war, US President Donald Trump has said.
Speaking on Friday evening, Mr Trump denied speculation he could visit Moscow for talks on 9 May - the day Russia celebrates its victory over the Nazis.
