A clip posted by a White House communications chief – and shared by Donald Trump – is the first ever to be flagged as “manipulated media” by Twitter.
Dan Scavino, the White House’s social media director, tweeted the clip of Joe Biden late on Saturday night.
In the clip, Mr Biden says: “Excuse me. We can only re-elect Donald Trump.”
However, the clip had been edited to remove some of the context of his speech in Missouri. Mr Biden actually said: “Excuse me. We can only re-elect Donald Trump if in fact we get engaged in this circular firing squad here.
“It’s gotta be a positive campaign so join us.”
The clip, which is still on the platform, was flagged by Twitter as “manipulated media” as part of its new policy to label some content which it feels is misleading, manipulated or synthetic – such as deep-fakes and edited clips.
President Trump, who is known for frequently calling out what he feels is fake news, retweeted the flagged clip shortly after the tweet went out.
Twitter has confirmed it tagged the clip, but a glitch means it does not appear when the tweet is searched for or clicked into and can only been seen when it appears organically in timelines, something the company says it is working on.
Mr Scavino defended the video, which has been watched more than six million times, saying: “The video is NOT manipulated.”
The video also remains on Facebook, where it has been given a similar tag, saying the video is “partly false information”.
Mr Biden’s campaign had previously accused Facebook of facilitating the “spread of vile lies”.
Social media companies are under pressure to better manage the spread of inaccurate information in politics ahead of November’s US election.
Bernie Sanders and Mr Biden remain in the race to become the Democrats’ nominee and ultimately take on Mr Trump for the White House.