Portugal head coach Fernando Santos and Juventus have both offered unwavering support to Cristiano Ronaldo after the five-time Ballon d'Or winner "firmly denied" a rape allegation.

Kathryn Mayorga has accused Ronaldo of sexual assault in a hotel in Las Vegas in 2009.

Ronaldo called the claims "fake news" before issuing a further statement on social media in which he strenuously denied the allegations and called rape "an abominable crime". 

Juventus posted on Twitter on Thursday offering their support to Ronaldo, who joined the Serie A champions from Real Madrid in July.

"Cristiano Ronaldo has shown in recent months his great professionalism and dedication, which is appreciated by everyone at Juventus," the club said in a statement.

"The events allegedly dating back to almost 10 years ago do not change this opinion, which is shared by anyone who has come into contact with this great champion."

Las Vegas police this week reopened an investigation into an alleged sexual assault from the same year as Ms Mayorga's allegation, but they have not named either the victim or any suspects.

Ms Mayorga's attorneys held a news conference on Wednesday in which they outlined the emotional impact they say their client has suffered as a result of her alleged assault. 

German publication Der Spiegel first reported the claims, prompting Ronaldo to stress his innocence.

"I firmly deny the accusations being issued against me," he posted on Twitter. "Rape is an abominable crime that goes against everything that I am and believe in. Keen as I may be to clear my name, I refuse to feed the media spectacle created by people seeking to promote themselves at my expense.

"My clear conscious [sic] will thereby allow me to await with tranquillity the results of any and all investigations."

Santos did not include Ronaldo in his squad for Portugal's upcoming matches against Poland and Scotland, announced on Thursday, the captain having also been absent for last month's games against Italy and Croatia.

The Portugal coach did not explicitly confirm the reasons for Ronaldo's omission, although when questioned in the news conference he also offered support to Ronaldo over the rape allegation.

"I believe in what the player said, two or three days ago, he considers rape an abominable act," Santos told reporters. 

“He's said that, again and again, that he's innocent and has nothing to do with it, with what he's being accused of.

"I know Ronaldo pretty well, so I believe completely in what he says. I know he wouldn't commit such a crime. I don't believe that."