A porn star who alleges she had an affair with Donald Trump is suing the US president for defamation after he attacked her claims in a tweet.

Stephanie Clifford, who goes by the stage name Stormy Daniels, filed the lawsuit in federal court in New York on Monday. At issue is a tweet Mr Trump made in which he dismissed a composite sketch that Ms Daniels says depicted a man who threatened her in 2011 to stay quiet about her alleged sexual encounter with Trump.

Mr Trump tweeted in April: "A sketch years later about a nonexistent man. A total con job, playing the Fake News Media for Fools (but they know it)!"

The lawsuit says the tweet was "false and defamatory," arguing that Mr Trump was speaking about Ms Daniels and that he "knew that his false, disparaging statement would be read by people around the world, as well as widely reported."

It also states that Ms Daniels has been "exposed to death threats and other threats of physical violence."