Twitter has used a new policy to identify misleading content for the first time on a post retweeted by US President Donald Trump that appeared to show Democratic Party rival Joe Biden endorsing his re-election.

The social media platform has struggled with removing deceptive and abusive content in the past, but has also faced accusations its curbs have suppressed some political views.

Last week it announced moderators would begin flagging media that had been "deceptively altered or manipulated," the same day it announced an expansion of a ban on hate speech.

A clip posted by White House social media director Dan Scavino appeared to show Biden telling a crowd: "We can only re-elect Donald Trump."

The footage was later retweeted by Trump and had been viewed nearly six million times by Monday. 

But the video cropped the end of Biden's sentence -- made during a recent campaign speech Missouri state -- where he was discussing the need for party unity in the wake of a bruising primary contest.

"We can only re-elect Donald Trump if in fact we get engaged in this circular firing squad here," Biden had said. 

Twitter appended a label to the video describing it as "manipulated media," prompting a denial from Scavino in a later tweet on Monday. 

The platform last month suspended dozens of accounts for spam-like posts supporting former Democratic presidential hopeful Michael Bloomberg, whose campaign hired hundreds of people to promote him on social media.