US President Donald Trump has said voters should send in their ballots as early as possible ahead of the 3 November presidential election and then go to the polls in person to make sure their votes are counted.

In a series of tweets, Mr Trump wrote: "... in order for you to MAKE SURE YOUR VOTE COUNTS & IS COUNTED, SIGN & MAIL IN your Ballot as EARLY as possible. On Election Day, or Early Voting, go to your Polling Place to see whether or not your Mail In Vote has been Tabulated (Counted)."

The tweets were posted a day after he suggested residents of North Carolina should cast their ballot once by post and again in person to test the voting system.

"Let them send it in and let them go vote," President Trump said in an interview with WECT-TV in Wilmington, North Carolina.

"And if the system is as good as they say it is then obviously they won't be able to vote" in person.

He has repeatedly asserted, without evidence, that postal voting - expanded by some states because of the coronavirus pandemic - would increase fraud and disrupt the November election, although experts say voter fraud of any kind is extremely rare in the United States.

Voting more than once in an election is illegal and in some states, including North Carolina, it is a crime not only to vote more than once but also to induce another to do so.

The North Carolina State Board of Elections confirmed this and said there are numerous checks to prevent people from double voting in the state.

After his comments raised eyebrows and the anger of Democrats, President Trump's campaign and the White House said he was not telling people to vote twice.

"The president is not suggesting anyone do anything unlawful," White House spokesperson Kayleigh McEnany told the Fox News channel.

"What he said very clearly there is make sure your vote is tabulated and if it is not then vote."

President Trump campaigned yesterday in North Carolina, known as a battleground state because of its population, can swing either to Republicans or Democrats and play a decisive role in presidential elections.

Ballots are due to be posted in North Carolina tomorrow.

The state's Attorney General Josh Stein, a Democrat, responded on Twitter, writing that President Trump, a Republican, had "outrageously encouraged" North Carolinians "to break the law in order to help him sow chaos in our election."

Mr Stein wrote: "Make sure you vote, but do NOT vote twice! I will do everything in my power to make sure the will of the people is upheld in November."

National opinion polls show Mr Trump trailing his Democratic rival Joe Biden.

Democrats accuse Republicans of trying to suppress the vote to help their side.

A record number of postal ballots are expected this year as people seek to avoid crowded polling places amid Covid-19.

Experts have cautioned such a surge will mean a winner may not be clear on election night given the time it will take to count and verify all the ballots.

Nearly one in four voters cast presidential ballots by mail in 2016.