US President Barack Obama has described GOP nominee Donald Trump's comments on the possibility of the billionaire ignoring the result of the November election as "dangerous."

During the third and final presidential debate, Trump told host Chris Wallace he would decide whether to accept the result on the day itself, adding "I'll keep you in suspense".

And the following day, Trump said he would pay attention to what the voters said, quipping: "I will accept the result – if I win".

"Of course, I would accept a clear election result, but I would also reserve my right to contest or file a legal challenge in the case of a questionable result," he added.

But at a Hillary Clinton rally held at Florida Memorial University, Obama told a crowd that Trump's comments were no joking matter, The Hill reported.

"This is just more than the usual standard lie," Obama said.

"That is not a joking matter. That is dangerous, because when you try to sow the seeds of doubt in other people's minds about the legitimacy of our elections, that undermines our democracy."

Barack Obama has criticised Donald Trump for suggesting he may not accept the election result.

Trump's comments have also been criticised by people from within his own party, with Republican senator John McCain speaking out against the suggestion a GOP candidate would not accept the result of the public vote.

"I didn't like the outcome of the 2008 election. But I had a duty to concede, and I did so without reluctance," McCain said in a statement seen by Politico.

"A concession isn't just an exercise in graciousness. It is an act of respect for the will of the American people, a respect that is every American leader's first responsibility."