US President Joe Biden refused Monday to back down on his weekend declaration in a major speech that Russian leader Vladimir Putin "cannot stay in power" -- arguing that he was voicing personal "outrage."

"I'm not walking anything back... I want to make it clear, I wasn't then, nor am I now, articulating a policy change. I was expressing moral outrage that I feel -- I make no apologies for my personal feelings," he told reporters at the White House.

Biden's remark -- delivered in Warsaw at the close of three days of marathon diplomacy on Saturday -- was seen as a gaffe by Republicans and some independent analysts concerned over a president going off-script when dealing with such a combustible conflict.

Biden said he was not worried that it would ratchet up tensions with Putin over Moscow's invasion of Ukraine, adding that he was "talking to the Russian people, telling them what we thought."

US President Joe Biden talks about the possibility of meeting Vladimir Putin as he announces his 2023 budget on March 28, 2022
US President Joe Biden talks about the possibility of meeting Vladimir Putin as he announces his 2023 budget on March 28, 2022 AFP / Nicholas Kamm

"I don't care what he thinks," Biden added. "This is guy who goes to the beat of his own drummer and the idea that he is going to do something outrageous because I called him for what he was and what he's doing, I think is just not rational."

Ukraine's government says that as many as 10,000 people may have died since the start of Putin's invasion more than a month ago.

Russian attacks near Kyiv have cut power to more than 80,000 homes, despite an apparent retreat in Moscow's war aims to focus on eastern Ukraine.

Biden left the door open to further diplomacy with Putin, however, saying US agreement to a meeting would depend "on what he wants to talk about."

"The question is, is there something to meet on that would justify him being able to end this war and being able to rebuild Ukraine," Biden told reporters.