The US election debates commission on Thursday turned down the Trump campaign's push to add a fourth televised clash with Democratic challenger Joe Biden.

The commission said that "three 90-minute debates work well to fulfill the voter education purposes the debates are intended to serve... (and it) remains committed to the schedule."

The first debate will take place September 29 in Cleveland, Ohio.

The event will inject drama into a presidential contest that has so far taken place largely online, as both candidates operate under the restrictions of the coronavirus shutdown.

President Donald Trump and his supporters have been pressuring for an additional earlier fixture.

They argue that with an increase in early mailed-in voting ahead of the November 3 election, voters should get a chance to see a Trump-Biden clash earlier.

Another factor is Trump's belief that his stage skills will favor him against Biden, giving him a chance to reverse worrying poll numbers with less than three months until election day.

"Move the First Debate up," the president tweeted on Thursday. "A debate, to me, is a Public Service. Joe Biden and I owe it to the American People!"

His campaign, represented by former New York mayor Rudy Giuliani, appealed to the non-partisan debates commission.

"For a nation already deprived of a traditional campaign schedule because of the COVID-19 global pandemic, it makes no sense to also deprive so many Americans of the opportunity to see and hear the two competing visions for our country's future before millions of votes have been cast," Giuliani wrote.

The commission responded that voters can just as easily wait for the debates before they mail their ballots.

"The debate schedule has been and will be highly publicized. Any voter who wishes to watch one or more debates before voting will be well aware of that opportunity," it said.

Trump and Biden will also debate on October 15 in Miami and October 22 in Nashville, Tennessee.

The vice presidential candidates will meet in Salt Lake City, Utah, on October 7.