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Officials at Mexico City's Chapultepec zoo unveiled a litter of Mexican grey wolf cubs on Wednesday.

The cubs are the first of their subspecies to be conceived by artificial insemination in Mexico.

Zoo officials described the birth of the endangered animals as a "worldwide success".

The Mexican grey wolf was almost wiped out by hunting, trapping and poisoning, but the US and Mexican authorities are trying to reintroduce them to the wild.

The last five known animals to survive in the US were captured in the late 1970s and bred in captivity.

A survey published in January by the US Fish and Wildlife Service showed there were 83 Mexican grey wolves living in the wild in Arizona and New Mexico.

Mexico began reintroducing wolves into the wild in 2011 and in July celebrated the birth of the first known litter of cubs born to released wolves.

The two cubs in Chapultepec zoo were born on 26 May to Ezita, a female from a zoo in Tamatan, Mexico, and Perkins, a male from the Endangered Wolf Center in Eureka, Missouri.

Zoo officials said they used artificial insemination because Ezita was already 11 years old and had had difficulty conceiving naturally in the past.