Zimbabwe has shipped 24 elephant calves to China, in a move that has angered animal rights groups who described the export as "extremely cruel".

"Some 24 elephants are on route to China as we speak after they were taken from their families in Hwange National Park," Johnny Rodrigues, chairman of the animal rights group Conservation Taskforce told AFP.

Rodrigues claimed that the calves were kept in cages at a secret location before being flown to China on Sunday night.

"The conditions in which these animals were exported are extremely cruel and we condemn the whole idea of separating baby elephants from their mothers."

Zimbabwean wildlife authorities have defended the sale as a measure to raise funds for conservation.

An elephant is estimated to fetch between $40,000 and $60,000.

In April, US-Canadian actress and animal rights activist Pamela Anderson petitioned the Zimbabwean government over the export of the animals to China.

She said the forced separation would cause them emotional stress.

The government says Zimbabwe is home to 80,000 elephants but can cope with only 42,000.

Rodrigues said authorities overestimated their numbers and urged the government to use alternative methods to control the population rather than resort to exports.

Zimbabwe is also battling high levels of poaching, with 300 elephants killed last year in Hwange park after poachers poisoned watering holes.