In his memoirs, Biden called Zelensky capricious and compared Putin to a boy.
The memoir "Promise Me, America" will be published on November 17, excerpts from which are published by the Wall Street Journal.
The former US president wrote that he felt the Ukrainian leader's acting background prepared him well for the role of supreme military commander, but that Zelensky "can be grumpy and stubborn in private and ungrateful as expected."
Biden also described the "nervous tone" of Vladimir Putin, who "liked to go wild during meetings and pretend to be indifferent like a teenager."
Biden also said that after Trump won the 2024 election, representatives of his team asked to sign a letter that would allow them to begin direct negotiations with Putin before the inauguration. But Biden refused because he didn't want to give them "carte blanche for what he expected to be some sort of capitulation at the expense of Ukraine."