WASHINGTON (AFP) - When Donald Trump complained about his microphone after his televised presidential debate Monday with Hillary Clinton, few people believed him - viewers had no trouble understanding his words.

But on Friday, the well-respected Commission on Presidential Debates said that in fact, the Republican candidate was right.

"Regarding the first debate, there were issues regarding Donald Trump's audio that affected the sound level in the debate hall," the commission said on its website, without elaborating.

In that terse phrase the independent group thus confirmed that Trump was right in denouncing the quality of his microphone, even if his initial complaint was met with widespread derision and disbelief.

"They gave me a defective mic. Did you notice that?" he asked reporters after the debate, which was viewed by a record-breaking 84 million people in the US alone.

"I mean, working that microphone was a hell of a lot more difficult than working crooked Hillary Clinton, that I can tell you," Trump said.

"I spent 50 per cent of my thought process working the mike."

He had wanted to pause the debate to address the problem, Trump said, but felt he could not. "How can I stop the show if I had 100 million people watching?" he said. Asked whether he was reconsidering participating in the next debate, scheduled for Oct 9, Trump did not answer directly.

"I want to do the next debate, but everybody is talking about the mike," Trump said.

Hillary Clinton, enjoying the moment after a debate in which Trump seemed to struggle as the evening wore on, quipped that "anyone complaining about microphones is not having a good night."

It was not clear how much difference the technical problems might actually have made. Trump's words were at all times audible on the several networks that carried the debate, though at times there seemed to be a slight echo.

That might, however, have been distracting for the candidate, particularly one who seems to thrive on audience reactions.