"If I start to do, to say anything that makes it possible to connect me to ongoing political debates in any allied country, then I will just weaken the Alliance," Stoltenberg said when asked about Biden during a news conference at NATO headquarters.

"I have had, and continue to have, a very good working relationship with President Biden," he said.

NATO next week holds a summit in Washington marking the 75th anniversary of its creation.

The gathering will take place as political clouds gather over 81-year-old Biden, who last Thursday put in a disastrous performance in a debate against Republican challenger Donald Trump.

The US president, speaking at a meagre volume, lost his train of thought at times and stumbled over words, sparking panic in his Democratic Party about his energy and mental acuity.

Biden, who has rebuffed subsequent suggestions he should stand aside for another, younger Democratic candidate, will give an interview on American television later Friday.

"I met him just a couple of weeks ago," Stoltenberg said.

"We had good meetings in the Oval Office. We made progress in the preparations for the upcoming NATO summit, on Ukraine, on deterrence, on defence and many other issues.

"And I welcome his strong personal leadership, on the support for Ukraine, on the support for NATO," he said.

The White House has said Biden will hold a news conference during next week's NATO summit.

The US presidential election takes place on November 5, with the winner to take office in January next year.