The Toronto Zoo says three newborn polar bear cubs have died.

Aurora, an 11-year-old female polar bear, gave birth to one female and two male cubs on Dec. 6, the zoo said in a news release on Monday.

One of the male cubs died shortly after it was born, and the two others died on Sunday. The zoo said it wasn't clear how the second two cubs had died and that tissue samples will be analyzed.

The zoo said that the silver lining in the deaths was that Aurora showed maternal instincts for the first time, after rejecting three cubs last year, only one of which survived.

"The fact that Aurora was doing everything right this time is a huge step in the right direction," said Maria Franke, the zoo's curator of mammals.

Polar bears are considered critically endangered, with between 20,000 to 25,000 wild polar bears worldwide — about 60 per cent of those in Canada.

"The information gained throughout Aurora's breeding has allowed us invaluable information that we can learn from and share with our counterparts," the zoo said.