A zoo in western Canada is mourning the loss of a rare Amur tiger after a staff blunder allowed the old cat to enter the cage and confront two younger felines.

"It is with great sadness that the Assiniboine Park Zoo announces that one of its Amur Tigers, Baikal, passed away," said a statement posted on the Winnipeg, Manitoba zoo's website overnight.

Baikal had entered a new tiger enclosure that housed two young male tigers, Samkha and Vasili. The brothers had been transferred from the Calgary Zoo to the Assininboine Park Zoo in January.

Officials said a gate had been "mistakenly left unlocked," allowing Baikal to pass through a corridor from his pen to the new enclosure.

"This resulted in a confrontation between Baikal and Vasili, one of the young tigers, that ultimately resulted in Baikal's death," the zoo said.

Amur tigers, also known as Siberian tigers, are one of the rarest sub-species of the endangered big cat.