The US sportswear giant - which has a contract worth a reported $1bn (£768m) with Ronaldo - said it would "continue to closely monitor the situation", the Associated Press reports.

EA Sports, which also has a contract with Ronaldo, made similar comments.

Ronaldo, 33, earlier "firmly" denied assaulting Kathryn Mayorga at a Las Vegas hotel in 2009.

Ms Mayorga, a 34-year-old US former teacher, was inspired to speak out by the #MeToo movement, her lawyer said.

"The MeToo movement and the women who have stood up and disclosed sexual assaults has given Kathryn a lot of courage," Leslie Stovall said.

What did Nike and EA Sports say?

In a statement, Nike said: "We are deeply concerned by the disturbing allegations and will continue to closely monitor the situation."

Meanwhile, EA Sports told the AP: "We have seen the concerning report that details allegations against Cristiano Ronaldo.

"We are closely monitoring the situation, as we expect cover athletes and ambassadors to conduct themselves in a manner that is consistent with EA's values."

In a separate development, Juventus - the Italian club Ronaldo joined from Real Madrid for £99.2m in July - came out in defence of its player.

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

How has Ronaldo responded?

The Portugal international had previously said the allegation, first reported in German magazine Der Spiegel, was "fake news".

On Wednesday, he issued a statement through his Twitter account:

Der Spiegel said Ms Mayorga had filed a report with Las Vegas police shortly after the alleged incident.

But the next year, she reportedly reached an out-of-court settlement with Ronaldo involving a $375,000 (£288,000) payment for agreeing never to go public with the allegations.

Her lawyers are now seeking to declare the non-disclosure agreement void.

In a lawsuit, Ms Mayorga says she met Ronaldo at the Rain Nightclub in the Palms Hotel and Casino, and that he raped her in his penthouse suite.

Mr Stovall said his client had suffered from major depression and considered suicide since the alleged assault almost a decade ago.

He said a psychiatrist had diagnosed Ms Mayorga with post-traumatic stress disorder.

Las Vegas police confirmed on Tuesday they had initially investigated a complaint in June 2009, but added they had no suspect in the case.

"At the time the report was taken, the victim did not provide detectives with the location of the incident or suspect description," a statement said.

"As of September 2018, the case has been reopened and our detectives are following up on information being provided," it added.

Ronaldo's lawyers have previously said they will sue Der Spiegel magazine over its reporting.