Top Republicans and Democrats pushed back hard Thursday against President Donald Trump's suggestion he might not accept defeat in the November election, warning him the United States was not "North Korea."

A day after the US leader refused to clearly guarantee a peaceful transfer of power, Republican Senate Speaker Mitch McConnell felt it necessary to assure American voters that the winner of the November 3 election would take office as planned in January.

The FBI, meanwhile, implicitly rejected Trump's suggestion that massive fraud was in the works with the surge in mailed-in ballots -- while it warned of disinformation about the issue.

Trump sparked outrage on Wednesday suggesting he might not honour the results of the election or treat mail-in ballots as legitimate.

Asked at a White House press conference whether if he is committed to the peaceful handover of power if defeated, Trump replied: "Well, we're going to have to see what happens."

"You know that I've been complaining very strongly about the ballots, and the ballots are a disaster," he said.

Trump repeated his claim that mailed ballots cannot be trusted in a radio interview early Thursday. Pressed on his remarks, White House spokeswoman Kayleigh McEnany told reporters: "The president will accept the results of a free and fair election."

Trump's remarks casting doubt on the transfer of power came as he significantly trails Democratic rival, Joe Biden, in most national polls on the presidential election.

McConnell, who wields significant power as the top Republican in Congress, felt it necessary to issue a statement seen as a veiled warning to Trump.

"The winner of the November 3rd election will be inaugurated on January 20th," McConnell tweeted. "There will be an orderly transition just as there has been every four years since 1792."

Other political leaders were more brusque. "Fundamental to democracy is the peaceful transition of power; without that, there is Belarus," Republican Senator Mitt Romney tweeted.

Nancy Pelosi, the Democratic House Speaker, said it was necessary to remind Trump, "You are not in North Korea, you are not in Turkey, you are not in Russia, President."

Senator Bernie Sanders, who Trump has assailed as an anti-American extreme leftist, excoriated Trump in a speech before the Senate.

"Under Donald Trump, we have a president who has little respect for our constitution or the rule of law," said Sanders, who earlier this year failed in a bid to win the Democratic presidential nomination.

Trump, he said, is "the first president in the history of this country to refuse to commit to a peaceful transition of power if he loses the election."

"What he is saying is that if he wins the election, that's great. But if he loses, it's rigged, because the only way, the only way, he can lose is if it's rigged."

On the campaign trail, Trump has repeatedly made unfounded allegations that the election could be rigged by Democrats taking advantage of a surge in voting by mail due to the Covid-19 pandemic.

On Wednesday, he asserted that the mailed-in votes will all be suspiciously cast for Biden, and should not be counted. "The ballots are out of control," he said. "Get rid of the ballots and you will have a very peaceful -- there won't be a transfer, frankly. There will be a continuation," he said.

Election officials have real concerns that there will not be a clear winner on the day after the election, as millions of mailed-in ballots take time to be delivered to local election offices and be tabulated.

The FBI and US intelligence have warned that instigators, domestic and foreign, could take advantage of that period to spread fake news about fraud, stirring up doubts about the electoral process.

However on Thursday FBI Director Chris Wray told a Senate hearing that they had not seen any coordinated effort to manipulate the election results, "whether it's by mail or otherwise."