Zelensky was "capricious, stubborn and not grateful enough" in private conversations with Biden, the former US president said in his memoir Promise Me, America.
In the passage published by the Wall Street Journal, Biden also writes that members of Trump's team, after his victory in the 2024 election, asked Biden to sign a letter authorizing them to negotiate directly with Putin before Trump takes office.
Biden then refused, "doesn't want to give them carte blanche for what would be some kind of capitulation at the expense of Ukraine."