Donald Trump has threatened to "close down" social media platforms after claiming they are biased against conservatives.
The US president had two tweets from his account flagged with a fact-check warning on Tuesday after he called postal voting "fraudulent".
Under the tweets, there is now a link which reads "get the facts about mail-in ballots" and guides users to a Twitter page with fact checks and news stories about Mr Trump's unsubstantiated claims.
In response, Mr Trump accused social media sites of having tried to prevent him from winning the 2016 presidential election and told them to "clean up your act".
"Republicans feel that social media platforms totally silence conservative voices," he wrote.
"We will strongly regulate, or close them down, before we can ever allow this to happen.
"We saw what they attempted to do, and failed, in 2016. We can't let a more sophisticated version of that happen again."
Mr Trump has previously claimed on several occasions, without evidence, that social media platforms are biased against conservatives.
In his latest tweets, he also reiterated his claims about postal voting.
"Just like 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," he wrote.
"Whoever cheated the most would win. Likewise, social media. Clean up your act, now."
Twitter has previously been criticised for not taking action against Mr Trump's account over a number of tweets that appeared to breach the platform's rules, including spreading misinformation or aiming abuse at individuals.
The firm has generally exempted political leaders from some of its rules, arguing that publishing controversial tweets from politicians encourages discussion.
However, it did update its policies last year, confirming that in future it would add warning labels to tweets from politicians that it found breached its rules.