The US president has claimed without evidence that mail drop boxes for voting are "not COVID-19 sanitised".

Donald Trump tweeted about Democrats' use of mail drop boxes.

Twitter has flagged one of Donald Trump's tweets as "violating" its policy for "making misleading health claims that could potentially dissuade people from participation in voting".

The US president has repeatedly criticised postal voting ahead of November's election, but in a tweet on Sunday he suggested without evidence that mail drop boxes would encourage fraud and put people at risk of contracting coronavirus.

"So now the Democrats are using Mail Drop Boxes, which are a voter security disaster. Among other things, they make it possible for a person to vote multiple times," he said.

The social media site later put a disclaimer on the tweet and limited how people can interact with it - meaning people can no longer like, reply or retweet it.

"Also, who controls them, are they placed in Republican or Democrat areas? They are not Covid sanitised. A big fraud!"

So now the Democrats are using Mail Drop Boxes, which are a voter security disaster. Among other things, they make it possible for a person to vote multiple times. Also, who controls them, are they placed in Republican or Democrat areas? They are not Covid sanitized. A big fraud!

— Donald J. Trump (@realDonaldTrump) August 23, 2020

But it said it may be "in the public's interest for the tweet to remain accessible".

Twitter pointed to its Civic Integrity Policy, which states that: "You may not use Twitter's services for the purpose of manipulating or interfering in elections or other civic processes.

"This includes posting or sharing content that may suppress participation or mislead people about when, where, or how to participate in a civic process."

Users who violate the policy can have their tweets deleted or accounts locked, or they could even be permanently suspended if there are "severe or repeated violations".

We placed a public interest notice on this Tweet for violating our Civic Integrity Policy for making misleading health claims that could potentially dissuade people from participation in voting. https://t.co/MA6E7mBpkm

— Twitter Safety (@TwitterSafety) August 23, 2020

Mr Trump was first given a fact-check label on one of his tweets in May.

Another of his posts was given a warning notice in June after the site said it violated policy against abusive behaviour.

Democrats have been promoting drop boxes as a convenient option for voters who do not want to use the US Postal Service, but Republican officials in other states have prevented their use.