Biden refers to Ukraine's Zelensky as 'President Putin'

US President Joe Biden on Thursday introduced Ukrainian President Volodymyr Zelensky as "President Putin" in a speech at a NATO summit in Washington.

"And now I hand over to the president of Ukraine, who has as much courage as he has determination - ladies and gentlemen, President Putin," Biden said.

Biden corrected himself a moment later, adding: "....to beat President Putin - President Zelensky!"

The apparent slip-of-the-tongue came at a time when US politics is becoming increasingly dominated by concerns about the 81-year-old president's mental capacity.

His performance is being watched closely, especially by his own Democratic Party that wants to defeat Republican Party nominee Donald Trump in November's presidential election.

When opening the summit with a speech at a ceremony marking NATO's 75th anniversary on Tuesday, Biden was able to read his speech from a teleprompter without any major slip-ups, though he occasionally slurred his words.

Later on Thursday, however, he is expected to face a press conference at the end of the NATO summit.

Here, he will have to react spontaneously - and without the help of the teleprompter.