Cristiano Ronaldo may have violated Italy's coronavirus preventive measures by traveling to Portugal while Juventus were in isolation, and returning to Turin this week after testing positive, Italy's sports minister Vincenzo Spadafora said on Thursday.

"Yes, I think so, if there were no specific authorizations from the health authority," Spadafora told Radio Uno when asked whether the 35-year-old Portuguese star had breached COVID-19 rules.

Ronaldo risks a fine after joining his national side last week despite Juventus being in isolation after two staff members tested positive.

The five-time Ballon d'Or winner tested positive on Monday, the day after Portugal drew 0-0 with France in Paris, and returned to Italy on his private jet on Wednesday.

Juventus president Andrea Agnelli said Thursday that his club had respected sporting protocol, referring to the "Ministries of Health and the Interior" to find out which rules Ronaldo had broken.

"Cristiano Ronaldo returned to Italy with a medical flight authorized by the competent health authorities at the request of the player and will continue his isolation at his home," Juventus said in a statement.

The entire Juventus team were back in isolation on Wednesday evening after U.S. midfielder Weston McKennie tested positive. 

Both McKennie and Ronaldo are in quarantine, for at least 10 days, and must test negative before rejoining their teammates.

"I think that, at the moment, the protocols in force for the sports championships, both for Serie A football and for the sports associations and clubs are valid as long as they are respected," said Spadafora. 

"And if there is someone who does not respect them, then the cases that we read in the news are created."

Under UEFA rules a player must provide evidence that he is no longer sick a week before a European match.

Ronaldo, who will miss Juventus's Champions League opener against Dynamo Kiev on October 20, must test negative on October 21 to play against Barcelona a week later.