Carrie Fisher is being treated in an intensive care unit after a serious medical emergency on a flight, according to her brother.

On Friday night, Todd Fisher told The Associated Press that his sister was receiving excellent care, but said that he could not classify her condition. He had earlier told the AP that she had been stabilized and was out of the emergency room.

In a subsequent interview, he said many details about her condition or what caused the medical emergency are unknown, and much of what had been reported was speculation.

TMZ and the Los Angeles Times described her condition as critical. A local ABC affiliate reported that Fisher is "on a ventilator."

In a subsequent interview with AP, her brother said she was being treated in intensive care, but that he could not classify her condition. He noted that much of what has been reported about Fisher is speculation.

"We have to wait and be patient," he said. "We have so little information ourselves."

According to several media reports, the actress Carrie Fisher suffered a heart attack on a flight Friday afternoon, according to media reports.

TMZ and the Los Angeles Times reported that the actress, best known for playing Princess Leia in the Star Wars film franchise, was on a flight from London to Los Angeles when she went into cardiac arrest. TMZ reports that the emergency occurred about 15 minutes before landing, and an EMT on the plane administered CPR before landing. The plane arrived just after 12:00 p.m. noon PT and Fisher, 60, was rushed by paramedics to the UCLA Medical Center. a nearby hospital. The LA Times reported that she is in critical condition.

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Fisher's daughter, Billie Lourd, was spotted at the hospital, as was Fisher's French bulldog, Gary.

The Los Angeles Fire Department confirmed that they responded to a call from the airport, but would not release any additional information, including the name, age or gender of the patient.

“At 12:11 pm the Los Angeles Fire Department responded to a call from LAX for a patient that was in cardiac arrest," Erik Scott, spokesman for the Los Angeles Fire Department, told USA TODAY. "Firefighter paramedics provided advanced life support and aggressively treated the patient whom was transported to a local hospital.”

People reports that Fisher was in London to film Season 3 of sitcom Catastrophe, which streams on Amazon in the USA. Co-star and co-creator Sharon Horgan shared a photo with Fisher on her Instagram last week, captioning it, "Me, General Leia and Kylo Ren's hand on the set of #catastrophe3." Horgan's co-creator, Rob Delaney, tweeted Wednesday that the third season of the show was "in the can."

Celebrities immediately took to social media to express their well-wishes for Fisher, including her Star Wars co-star Mark Hamill.

Fisher has been making waves recently promoting her latest memoir, The Princess Diarist, based on the diaries she kept while starring in the original Star Wars film as a 19-year-old. In the book she reveals, among other things, that she and her co-star Harrison Ford had an affair on the set that they kept secret for decades. She reprised her world-famous role in the seventh film of the sci-fi series, The Force Awakens, which hit theaters in December of 2015. She also appears in the upcoming Episode VIII, which is in post-production and set for release Dec. 17, 2017.

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The actress, who is the daughter of singer Eddie Fisher and singer/dancer/actress Debbie Reynolds, is also known for roles in Blues Brothers and Soapdish. She's appeared in guest-roles on TV shows like The Big Bang Theory, 30 Rock and most recently Bravo's Girlfriend's Guide to Divorce and Catastrophe.

Fisher's reps did not immediately respond to USA TODAY's request for comment.

Contributing: The Associated Press