Boxing great Muhammad Ali is in hospital in Arizona with what sources familiar with his condition say may be more serious problems than before.
Two people said Ali, 74, was fighting respiratory issues complicated by the Parkinson's that he was diagnosed with in the 1980s.
They spoke separately in describing Ali's condition that they say is concerning to family members.
A spokesman for Ali sent out a release earlier yesterday saying the former heavyweight champion is being treated at an unidentified hospital in the Phoenix area for a respiratory issue. Spokesman Bob Gunnell said that the 74-year-old was in a fair condition and a brief hospital stay was expected.
Ali has been admitted to hospital several times, 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 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 Centre in his home town 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 probably caused by the thousands of punches Ali took during his career, where he travelled the world meeting opponents in big fights.
An iconic figure who was at one point arguably the most recognised person in the world, Ali has lived quietly in the Phoenix area with his fourth wife Lonnie, whom he married in 1986.
News of his admission to hospital brought well wishes from boxers and others on Twitter, including Sugar Ray Leonard, who modelled his career after Ali.