Washington (AFP) - US President Donald Trump on Thursday renewed his call for supporters to try to vote twice on November 3, a potentially illegal act that he claimed is the only way to be sure that a ballot is counted.

Trump said Americans should first try to vote by mail, if that option is offered in their state, then also go to the polling station on election day to check that their ballot has been counted -- and, if not, vote again.

"YOU ARE NOW ASSURED THAT YOUR PRECIOUS VOTE HAS BEEN COUNTED," he said in a Twitter thread.

Intentionally voting twice is illegal.

Facebook and Twitter, which are under growing pressure not to allow misinformation ahead of the election, put warning labels on the president's posts about the matter.

"We placed a public interest notice on two Tweets in this thread for violating our Civic Integrity Policy, specifically for encouraging people to potentially vote twice," Twitter said.

However, White House spokeswoman Kayleigh McEnany said Trump had been taken out of context and just wanted voters to verify that their first vote had been registered.

He "does not condone unlawful voting," she said.

Whatever Trump meant, the tweets were his latest assault on the credibility of mail-in voting, which is being rapidly expanded across the country in response to fears of Covid-19 contagion in crowded polling stations.

Despite Trump's relentless challenges, the mail-in practice is already widespread and largely trouble-free in the United States. 

He himself uses the absentee mail-in option to vote in Florida while living in the White House.

While Trump says that increased mail-in voting will allow mass ballot tampering and rigging of the result, elections experts say there is no evidence of significant danger.

However, polls show Democrats are far more likely than Republicans to vote by mail, leading Trump's opponents to fear that he is trying to suppress Biden's vote -- or set the stage for disputing the results.

Election day worries

Trump first aired the idea of casting a precautionary second vote on Wednesday during a trip to North Carolina.

"If you get the unsolicited ballots, send it in, and then go -- make sure it counted. And if it doesn't tabulate, you vote," he said. "So, send it in early, and then go and vote."

The recommendation prompted the election board in the state, which Democrat Joe Biden hopes to snatch from Trump, to issue a statement pointing out: "It is illegal to vote twice in an election."

"Soliciting someone to do so also is a violation of North Carolina law," the board's director, Karen Brinson Bell, added.

Polls currently show Biden on course to defeat Trump.

But experts warn that the country faces a confusing and potentially tense aftermath of the voting on November 3, due to the unusual circumstances.

Given the larger than ordinary number of mailed-in ballots, few expect there will be a clear-cut result that evening, with mailed ballots possibly taking days if not weeks to finish counting.

Under one scenario, Trump could be shown to have won on the basis of ballots cast during the day, but mail-in ballots, once counted days later, would then put Biden over the top.

Trump has repeatedly refused to declare he will accept the results of the election.