U.S. President Joe Biden on Thursday closed a three-day NATO summit in Washington by promising to “keep NATO strong,” in a press conference that focused heavily on U.S. politics and concerns about his mental fitness.

The 81-year-old U.S. leader is facing calls to let another candidate from his Democratic Party replace him in this year’s presidential race due to fears that he is suffering a mental decline.

The opposing Republican Party is expected to nominate former president Donald Trump, 78, at its convention, which begins on Monday in Milwaukee, Wisconsin.

“My predecessor has made it clear he has no commitment to NATO,” Biden said.

“He’s made it clear that he would feel no obligation to honour Article 5″ of the NATO treaty, which commits all allies to respond if one is attacked.

“But I made it clear a strong NATO is essential to American security. And I believe the obligation of Article 5 is sacred,” Biden contrasted.

“I will not bow down to Putin. I will not walk away from Ukraine. I will keep NATO strong,” the U.S. leader continued.

Biden’s speech followed a three-day summit in which NATO leaders announced further military support for Ukraine’s efforts to repel the Russian invasion, including air-defence systems and a pledge to spend 40 billion euro (or 43 billion dollars) in the next year.

They also declared Ukraine’s future path to NATO membership “irreversible.”

The United States and Germany also announced on Wednesday that the former would station long-range cruise missiles in “episodic deployments” in Germany to strengthen NATO’s deterrence in Europe.

But Biden’s closing press conference was dominated by the forthcoming presidential election in his country, and whether he should persist in running for re-election.

When asked whether he’d still be able to “deal with” Russian President Vladimir Putin and Chinese President Xi Jinping three years from now, Biden said he would not talk to Putin until the Russian leader changed course.

“I’m dealing with Xi right now and (I’m in) direct contact with him.

“I have no good reason to talk to Putin right now. There’s not much that he is prepared to do in terms of accommodating any change in his behaviour.

“I’m not ready to talk to Putin unless Putin’s ready to change his behaviour,” he said.

At times during the hour-long news conference, Biden struggled to express himself coherently, and in one instance appeared to confuse Trump with Vice-President Kamala Harris.

When asked for his opinion of Harris’ ability to beat Trump, he said: “Look, I wouldn’t have picked Vice President Trump to be vice president, did I think she was not qualified to be president” (sic).

He went on to say, “I think I’m the most qualified person to run for president.”

This followed an earlier gaffe in a speech after a meeting with Ukrainian President Volodymyr Zelensky, when Biden introduced the Ukrainian leader as “President Putin.”

“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!”

In a post-summit news conference shortly before Biden’s, Britain’s newly-appointed Prime Minister Keir Starmer was repeatedly asked about Biden’s mental fitness.

Starmer said Biden “deserves credit” for the summit’s achievements.

“We have a bigger NATO, and more countries; we have a stronger NATO. We have a real sense of resolve,” Starmer said.

“President Biden led through all of that,” he added.

Earlier in the day, Finnish President Alexander Stubb told reporters he had no concerns about Biden’s capabilities.

“I have absolutely no concerns about the capacity of the current president of the United States to lead his country and to lead our fight for Ukraine and to lead NATO,” Stubb told reporters.

“I’ve had the opportunity to speak to President Biden on many occasions during the past 48 hours.

“We are human beings, when we’re treated in the public eye, we’re never as good as we sometimes look, and we’re never as bad,” Stubb added.