Twitter has used its new "manipulated media" tag for the first time on an edited video of a speech by Joe Biden.
The tag, introduced on 5 March, tells readers when a picture or video has been "significantly altered or fabricated".
The footage of Mr Biden, posted by Dan Scavino, White House director of social media, claimed to show the Democrat calling for people to vote for President Donald Trump, a Republican.
The video was shared by Mr Trump twice.
In the clip, Mr Biden appeared to tell a crowd in Kansas City: "We can only re-elect Donald Trump."
However, it had been edited to remove key parts of the sentence.
Mr Biden, a leading candidate for the Democratic presidential nomination, actually said: "We can only re-elect Donald Trump if in fact we get engaged in this circular firing squad here. It's got to be a positive campaign."
Mr Scavino shared the video in a tweet on Saturday. The president then re-tweeted it, telling his followers: "I agree with Joe!"
The video was also shared on Facebook.
A spokesperson for Facebook told the BBC: "Fact-checkers rated this video as partly false, so we are reducing its distribution and showing warning labels with more context for people who see it, try to share it, or already have.
"As we announced last year, the same applies if a politician shares the video, if it was otherwise fact checked when shared by others on Facebook."
Did the Twitter warning work?
Twitter's manipulated media policy covers content that is "likely to impact public safety or cause serious harm" including material that has been heavily edited. Under the policy, Twitter may remove a tweet, reduce its visibility, or label it as doctored.
However, the social media giant has been criticised for taking at least 18 hours to place a manipulated tweet tag on Mr Scavino's post.
In that time, over five million people had viewed the video on the site alone.
The tag also failed to appear when people searched for the tweet. Twitter's spokeswoman Katie Rosborough told the Washington Post that the tag was appearing in individuals' timelines and the company was working on a fix.
Mr Scavino has defended the video, claiming it was not manipulated. He has also shared tweets defending him that allege the video was "clipped, not edited" and "simply shortened".
Greg Schultz, Joe Biden's campaign manager, called Facebook's lack of action in labelling manipulated footage a "national crisis", in a statement to CNN.
What are the new rules?
Twitter uses these factors to consider whether a tweet should be labelled or removed:
- Has the content been edited in a manner that alters its composition, sequence, timing or framing?
- Has any visual or auditory information been added or removed?
- Has media depicting a real person been fabricated or simulated?
- Is the media being shared in a deceptive manner?
- Is it likely to impact public safety or cause serious harm?