Donald Trump stepped up his attacks Friday on a former Miss Universe, urging the public to check out an alleged "sex tape" of the young woman who claims the billionaire bullied her and has turned supporter of Hillary Clinton.

In a pre-dawn Twitter storm, the Republican presidential candidate accused his Democratic rival of helping Alicia Machado get US citizenship so she could "use her" in this week's presidential debate.

Addressing his nearly 12 million followers, Trump charged that the Venezuela-born Machado, who won the Trump-owned beauty pageant in 1996, had "duped" the Clinton campaign.

"Did Crooked Hillary help disgusting (check out sex tape and past) Alicia M become a U.S. citizen so she could use her in the debate?" Trump tweeted.

"Wow, Crooked Hillary was duped and used by my worst Miss U. Hillary floated her as an "angel" without checking her past, which is terrible!" he wrote.

"Using Alicia M in the debate as a paragon of virtue just shows that Crooked Hillary suffers from BAD JUDGEMENT! Hillary was set up by a con," read a third tweet.

According to the fact-checking website Snopes, the sex tape allegation appears to refer to Machado's appearance on a television reality show in which she is shown in bed with another participant, but under the covers.

Machado was also pictured in Playboy magazine. But a pornographic video circulated in recent years purporting to star the former Miss Universe was debunked as a fake.

The tweets were continuing fallout from Monday's brass knuckles presidential debate, when Clinton assailed her rival for publicly humiliating Machado after she gained weight following her beauty pageant win.

Trump owned the Miss Universe beauty contest until last year.

His brutal criticism of Machado's weight -- including TV images repeatedly re-broadcast this week showing her being put through the paces of a weight-loss exercise regimen in front of television cameras -- is seen as potentially damaging with both women and Latino voters.

Machado accused him of having verbally abused her, calling her "Miss Piggy" for her post victory weight gain and "Miss Housekeeping" -- an apparent reference to her Latina heritage.

The former beauty queen has seen her profile raised considerably by the controversy.

During a news conference call earlier this week, speaking in a mix of Spanish and English, Machado heaped contempt on Trump for his past mistreatment.

"He doesn't remember me, he doesn't remember many things that he did to me, or that he told me," Machado told reporters.

"It was evident that maybe he doesn't remember an 18-year-old girl, whom he insulted and offended and mistreated a thousand times. But I'll always remember," she said.