LOS ANGELES (AFP) - A young American woman with terminal cancer has committed suicide, following promises to do so that had triggered shock and controversy over the right to die.

Brittany Maynard, a 29-year-old brain cancer sufferer, made headlines earlier this month when a video of her making her suicide threat went viral and was seen by millions of Web users.

"Goodbye to all my dear friends and family that I love. Today is the day I have chosen to pass away with dignity in the face of my terminal illness, this terrible brain cancer that has taken so much from me... but would have taken so much more," she wrote in a message circulated widely on social media.

"The world is a beautiful place, travel has been my greatest teacher, my close friends and folks are the greatest givers. I even have a ring of support around my bed as I type... Goodbye world. Spread good energy. Pay it forward!"

In January, Maynard was given six months to live and was told her death would be painful because of the aggressive nature of her cancer.

On Oct 6, she launched an online video campaign with Compassion & Choices, an end-of-life choice advocacy organisation, to fight for expanding death with dignity laws nationwide.

"For people to argue against this choice for sick people really seems evil to me," she told People magazine in the United States. "They try to mix it up with suicide and that's really unfair, because there's not a single part of me that wants to die. But I am dying."

She said she gradually arrived at her decision to die with dignity after realising that no patient had ever survived the disease.

Doctors removed as much of the tumor as they could, but it came back larger than ever two months later, she told the magazine, adding that she gave up chemotherapy or radiation on the grounds that the treatment would compromise the quality of life.

"They didn't seem to make sense for me," she said, because of "the level of side effects I would suffer and it wouldn't save my life. I've been told pretty much no matter what, I'm going to die."

She had been trying for a first child with her husband Dan Diaz, 43, at the time, but gave up due to her disease.

Maynard and her husband, who had just married when she began having severe headaches, moved from their home in California to Oregon, one of a handful of US states with a "right-to-die" law. A doctor could therefore prescribe her the medication she needs to end her own life, surrounded by her family in the bedroom she shares with her husband.

On Thursday, she released a new video in which she said she might temporarily delay her appointment with a self-administered cocktail of potentially deadly drugs. But the delay was a short one.

Her story has made headlines around the world, and she was featured on the cover of last week's People.

Maynard had in recent weeks and months been working to tick off items on a "bucket list" of what she wanted to do - including travelling to the Grand Canyon last week.

Maynard said she was deeply touched by the "outpouring of support" she got after going public with her diagnosis and her decision.

"I want to thank people for that, for the words of kindness, for the time they've taken in personal ways," she told People.

"For me what matters most is the way I'm remembered by my family and my husband as a good woman who did my best to be a good wife and a good daughter," she said.