US President Donald Trump warned Friday that the results of the  November 3 presidential election could take weeks or months to determine.

Amid concerns that a wave of mail-in ballots could overwhelm the post office and local election bodies, Trump suggested the traditional election-night verdict could be delayed. 

"You'll never have an election count on November 3," Trump said in a speech to the Council of National Policy, a conservative activist group.

"You're not going to be able to know the end of this election, in my opinion, for weeks, months, maybe never," he said.

The prospect of a slow count of an expected 50 million votes cast by mail, one consequence of the coronavirus pandemic, has raised concerns of political turmoil and mischief-making, and legal challenges that could further delay a result, for the presidential as well as congressional races.

Trump, who polls show is trailing challenger Joe Biden, alleged that Democrats were promoting universal mail-in voting to manipulate the results. But he suggested a systemic problem as well.

"We're not prepared for this, 51 million ballots. It will be a tremendous embarrassment for the country," he said. "This is a very serious problem for a democracy."

On Wednesday, a top US official for election security said his biggest worry is of outside interference in a likely count of the votes the day after the November 3 election.

While meddling by Russia, China, Iran and others in the run-up to the poll is a concern, "I'm worried about Election Day on," said Bill Evanina, director of the National Counterintelligence and Security Center.

He said external actors could use hacks like ransomware and other cyberattacks against the infrastructure for delivering, counting and transmitting the votes.

"We need to prepare as a nation that the election will not be decided on November 3," he said.

"I'm worried about ransomware attacks. I'm worried about cyberattacks. I'm worried about the inability of people to vote because of cyber penetrations and ransomware."

"So for me I worry about not up to the election, from the influence perspective, I'm worried about the interference perspective come November 3, 4 and all the way through November," he said.