Muhammad Ali hospitalized in Phoenix area

In this Sept. 27, 2014, file photo, Muhammad Ali is shown before the Ali Humanitarian Awards ceremony in Louisville, Ky. A spokesman for boxing great Muhammad Ali says the former heavyweight champion is being treated in a hospital for a respiratory issue. Spokesman Bob Gunnell said Thursday, June 2, 2016, that Ali is being treated by doctors as a precaution. (AP Photo/Timothy D. Easley, File)

Boxing great Muhammad Ali is hospitalized in the Phoenix area with what two people familiar with his condition say may be more serious problems than his previous hospital stays.

The people told The Associated Press on Thursday that Ali is fighting respiratory issues that are complicated by the Parkinson's that he was diagnosed with in the 1980s. The two spoke separately in describing Ali's condition that 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 sent out a release earlier Thursday saying the former heavyweight champion is being treated at an unidentified hospital for a respiratory issue. Spokesman Bob Gunnell said that the 74-year-old was in fair condition, and that a brief hospital stay was expected.

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.

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.

Ali has suffered from Parkinson's for three decades, most famously trembling badly while lighting the Olympic torch in 1996 in Atlanta. Despite the disease he kept up a busy appearance schedule until recently, though he has not communicated verbally in public for years.

Doctors say the Parkinson's was likely caused by the thousands of punches Ali took during his career, where he traveled the world meeting opponents in big fights.

An iconic figure who was at one point arguably the most recognized person in the world, Ali has lived quietly in the Phoenix area with his fourth wife, Lonnie, who he married in 1986.

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

''Prayers & blessings to my idol, my friend, & without question, the Greatest of All Time (at)MuhammadAli ! (hash)GOAT,'' Leonard wrote.