Muhammad Ali has been hospitalized and is being treated for a respiratory issue, family spokesman Bob Gunnell said in a statement Thursday.

"He is being treated by his team of doctors and is in fair condition," Gunnell said in a statement datelined from Phoenix, where Ali lives. "A brief hospital stay is expected. At this time, the Muhammad Ali family respectfully requests privacy."

In May 1965, Muhammad Ali was the subject of an iconic photograph, taken after he knocked down Sonny Liston in the first round of their fight. AP Photo/John Rooney

The Associated Press, citing sources it did not identify because they were not speaking on behalf of the family, said Thursday night that Ali's condition may be more serious than it was during his previous hospital stays.

Ali, 74, suffers from Parkinson's disease. The legendary three-time heavyweight champion has been hospitalized a few times in recent years, including twice between late 2014 and early '15 with a severe urinary tract infection initially diagnosed as pneumonia.

Ali has looked increasingly frail in public appearances in recent years, including on April 9 when he wore sunglasses and was hunched over at the annual Celebrity Fight Night dinner in Phoenix, which raises funds for treatment of Parkinson's. His last formal public appearance before that was in October, when he appeared at the Sports Illustrated Tribute to Muhammad Ali at The Muhammad Ali Center in his hometown of Louisville, Kentucky, along with former opponents George Foreman and Larry Holmes.

News of his hospitalization brought well wishes from boxers and others on Twitter, including Sugar Ray Leonard, who modeled his career after Ali's.

Prayers & blessings to my idol, my friend, & without question, the Greatest of All Time @MuhammadAli ! #GOAT — Sugar Ray Leonard (@SugarRayLeonard) June 2, 2016

Information from The Associated Press was included in this report.