Muhammad Ali remained hospitalized Friday in the Phoenix area, battling respiratory problems serious enough to draw family members to his bedside.

The 74-year-old boxing great's respiratory issues have been complicated by the Parkinson's that he was diagnosed with in the 1980s, two people told The Associated Press a day earlier.

The two spoke separately in describing Ali's condition as being very concerning to family members. They declined to be identified because they were not speaking on behalf of the family.

Get Breaking News Delivered to Your Inbox

Several of Ali's daughters reportedly flew to Phoenix late Thursday and early Friday to be with their father. Several of his children did not respond to messages from the AP Friday.

"It's extraordinarily grave," a source close to the family told Reuters. "It's a matter of hours."

The source said he had spoken with Ali's wife, Lonnie, and that funeral arrangements "are already being made."

Ali's daughter Laila, herself a former boxer, posted an old photo to Facebook on Friday of her father holding her daughter, Sydney.

"Thank for all the love and well wishes," Laila Ali wrote. "I feel your love and appreciate it!!"

"Laila's number one priority is her father's well-being. She truly appreciates the outpouring of love for her family, as she spends quality time with her dad," her business manager, Amber Noble Garland, told CBS News in a statement.

A spokesman for the former heavyweight champion said in an email Friday that there was no update on his condition.

Ali's longtime Parkinson's doctor declined comment when reached by the AP Thursday night.

"I can't really say much more than what's in the papers," said Dr. Abraham Lieberman of the Barrow Neurological Institute in Phoenix.

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, including 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 spoken in public for years.

Doctors say the Parkinson's likely was caused by the thousands of punches Ali took during a career in which he traveled the world for big fights.