US President Joe Biden was defiant against suggestions he is in cognitive decline ahead of a major NATO summit on Tuesday.

In a surprise call-in to the cable news show "Morning Joe" on Monday, Biden said he was getting "frustrated" by calls for him to pull out of the presidential race in favor of a different Democratic candidate.

He added that anyone was welcome to challenge him for the nomination at the party convention in August if they thought voters would support them.

In a letter to fellow party members, Biden wrote: "I wouldn't be running again if I did not absolutely believe I was the best person to beat Donald Trump in 2024...We had a Democratic nomination process and the voters have spoken clearly and decisively."

White House rejects Parkinson's claims

The New York Times has reported that Biden had repeatedly been visited by a neurologist specializing in Parkison's disease. 

But White House Press Secretary Karine Jean-Pierre refused to confirm the report during a news briefing on Monday.

"Has the President been treated for Parkinson's? No. Is he being treated for Parkinson's? No, he's not. Is he taking medication for Parkinson's? No," she said.

Biden's personal physician, Kevin O'Connor, said in a letter that Biden "has not seen a neurologist outside of his annual physical." 

Biden was expected to use the NATO summit in Washington to press his case after a television debate against presumptive Republican nominee Trump in June left voters questioning if age was taking its toll on the president.

What is the NATO summit about? 

On top of celebrating the alliance's 75th anniversary, the 3-day meeting will focus on the war in Ukraine as well as countering growing Chinese influence on the world stage.

The leaders of non-member states New Zealand, Japan and South Korea will attend the summit for the third year in a row amidst increasing Chinese military presence in the Pacific.

Ahead of the meeting, outgoing Secretary-General Jens Stoltenberg announced a $43 billion aid package to Ukraine, a reduction from previous packages that is expected to decrease further the following year.

However, the NATO Security Assistance and Training for Ukraine (NSATU) program, which was approved in June, is set to cement assistance for Kyiv in the future in the case of a Trump presidency.

Trump has suggested that he will be less generous to Ukraine than Biden if elected in November.

es/lo (AP, AFP, Reuters)