The White House Correspondents' Association said on Thursday CNN has rejected multiple requests to include White House pool reporters inside the studio during the first presidential debate between incumbent Joe Biden and Republican rival Donald Trump.

The press pool, made up of representatives of major news organizations including Reuters, accompanies the president on foreign and domestic trips and normally has access to any event where he speaks or appears in public, with the goal of keeping the U.S. public informed.

It is extremely rare for them to be barred from an event in the United States.

"WHCA is deeply concerned that CNN has rejected our repeated requests to include the White House travel pool inside the studio," Kelly O'Donnell, president of the White House Correspondents' Association, said in a statement.

"The pool is there for the 'what ifs?' in a world where the unexpected does happen," she said, and to provide "context and insight by direct observation and not through the lens of the television production," she said.

"These reporters are there to see what is said and done when the microphones and cameras are off, and provide independent observation, she wrote, with duties "separate from the production of the debate as a news event."

O'Donnell said both the Biden and Trump campaigns agreed to the WHCA's request.

CNN has only agreed to allow one White House print pool reporter to enter the studio during a commercial break to "briefly observe the setting."

The network will also allow still photographers from other outlets to cover the candidates inside the studio and will provide a television feed of the debate to other networks.

CNN has put in place many other rules for the first showdown including two commercial breaks, no props and muted microphones except when the candidates are recognized to speak.

The network said in a statement, opens new tab it is a member of the WHCA and respects the role the group plays to further press freedom and access. But the debate was being held without an audience in a CNN studio and is closed to the press, CNN said.

"Precedent matters for future debates," O'Donnell said, alluding to Biden and Trump's face-off again in September.

The National Association of Black Journalists has also asked CNN to accredit reporters from local Black-owned news organisations, after none of Atlanta's Black news groups got credentials to be on-site for the debate.