Boxing legend Muhammad Ali is hospitalized in Arizaona with what two people familiar with his condition say might be more serious problems than his previous hospital stays.

The sources told Associated Press on Thursday that Ali is fighting respiratory issues that are complicated by the Parkinson's he was diagnosed with in the 1980s.

The two spoke separately in describing Ali's condition, which they say is concerning to family members. The sources declined to be identified because they were not speaking on behalf of the family.

A spokesman for Ali released a statement saying the 74-year-old former three-time heavyweight champion is being treated at an unidentified Phoenix-area hospital for respiratory problems and was listed in "fair" condition.

Ali has been hospitalized several times in recent years, most recently in early 2015 when he was treated for a severe urinary tract infection initially diagnosed as pneumonia.

He 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 the 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.

Ali has been battling Parkinson's for three decade. His arm trembled badly while lighting the Olympic torch in 1996 in Atlanta.

Doctors say the Parkinson's was likely caused by the thousands of punches Ali took during his pro career from 1960-81, which saw notch a record of 56-5, with 37 knockouts.