US President Joe Biden on Friday acknowledged age and bad performance in the presidential debate against Donald Trump, but vowed to win the November election.

"I know I'm not a young man. I don't walk as easy as I used to. I don’t speak as smoothly as I used to. I don’t debate as well as I used to, but I know what I do know — I know how to tell the truth," Biden told supporters at a rally in Raleigh, North Carolina, his first campaign stop after his stumbling performance the day prior against Trump.

"I would not be running again if I didn't believe with all my heart and soul that I could do this job. The stakes are too high," he said.

Biden's rambling answers and appearing to lose his train of thought in the debate heightened voter concerns that he might not be fit to serve another four-year term.

He told reporters following the debate that it is "hard to debate a liar," referring to Trump, and said the ex-leader lied 26 times.

Asked if he was suffering from a cold, Biden said: "I have a sore throat."

The White House later confirmed that Biden had a cold and sore throat during the debate. "He had the cold and had a sore throat. Again, he tested negative for Covid, and then we moved on," spokeswoman Karine Jean-Pierre told reporters.

Biden's Campaign Communications Director Michael Tyler said the strategy "doesn't change at all" after the debate.

Asked whether the US president is committed to appear at the debate with Trump in September, Tyler said: "Joe Biden will be there on September 10. We'll see what Donald Trump does."