US President Donald Trump on Wednesday threatened to close down social media platforms after Twitter labelled two of his tweets "unsubstantiated" and accused him of making false claims.

"Republicans feel that Social Media Platforms totally silence conservatives voices. We will strongly regulate, or close them down, before we can ever allow this to happen," Trump tweeted.

Twitter targeted two tweets the president posted on Tuesday in which he contended without evidence that mail-in voting would lead to fraud and a "Rigged Election."

Under the tweets, Twitter posted a link which read "Get the facts about mail-in ballots" -- a first for the social network which has long resisted calls to censure the US president over truth-defying posts.

Trump pushed back again on Wednesday saying "we can't let large scale Mail-In Ballots take root in our Country. It would be a free for all on cheating, forgery and the theft of Ballots. "Whoever cheated the most would win. Likewise, Social Media. Clean up your act, NOW!!!!"

The president also accused social media platforms of interfering in the last election, saying "we saw what they attempted to do, and failed, in 2016."

"We can't let a more sophisticated version of that happen again."

The president has long used Twitter as a platform to spread abuse, conspiracy theories, false information and insults to his 80 million followers. Before being elected in 2016, he built his political brand by supporting the "birther" lie that Barack Obama, America's first black president, was not born in the United States and therefore was not eligible to be president.

And he has recently ignited another storm with an attempted character assassination of MSNBC TV host Joe Scarborough by spreading the baseless rumour he murdered an aide.