U.S. President Joe Biden, his aides and Western allies are scrambling to explain his remark that Russian leader Vladimir Putin could not remain in power because they do not want to escalate conflict between Washington and Moscow, officials said.

The nine-word line, at the end of a 27-minute speech in Warsaw on Saturday, has distracted from what some observers regard as the best piece of rhetoric of Biden's presidency. It made foreign allies uneasy at the end of an otherwise successful trip aimed at uniting allies against Russia, and has raised fresh questions about the United States' long-term strategy for its former Cold War foe.

"For God's sake, this man cannot remain in power," Biden said in the Polish capital after condemning Putin's month-long war in Ukraine at length.