WASHINGTON -- An animal rights group is demanding that a 40-year-old Asian elephant be moved to an animal sanctuary, claiming that the National Zoo is mistreating the animal.

Members of the group "Friends of Toni" said the elephant is being crippled in captivity, a condition that may become life-threatening.

"She's got her good days and her bad days. But I've been watching her a lot and she is deteriorating, losing weight, and leaning more and more," said Amy Mayers, a member of the animal rights group. "She's eventually going to die. I know all animals will die but this is a premature death. She's only 40-years old and in the wild she would do well."

The National Zoo has faced heavy criticism for its care of some animals in recent years. A congressional panel looked into care at the zoo after several animal deaths in 2003.

Most recently, veterinarians had to euthanize a 2-year-old giraffe, which was losing a battle with cancer.

Zoo officials said the allegations about Toni the elephant, which is popular with visitors, are not true.

"Toni is getting the best care possible. She's surrounded by her family of elephants and surrounded by her keepers," said assistant curator Tony Barthel.

Barthel admits the elephant suffers from arthritis, but claims the condition stems from a serious leg injury that happened before Toni came to the zoo.

"It changes the way they stand over time and subsequently, she's developed arthritis from standing strange and not the way she was meant to stand," said Barthel.

The zoo has recently added some things to make Toni a little more comfortable, such as new sand and a tire to lean on. Zoo officials said they also plan to add heaters to Toni's habitat soon.

However, Friends of Toni members said the additions are not enough. They think the elephant would be better off in an animal sanctuary.

Zoo officials said they doubt a move would help.

"There's no evidence to suggest that she would benefit from leaving. The only thing I can imagine is that she would come to harm," said Barthel.

Officials said Toni is responding to treatment and could live for several more years.