Seven black rhinos have died in unclear circumstances at the Tsavo East National Park in southern Kenya
Kenya Wildlife Service (KWS) has refused to comment on the rhinoceros deaths but, speaking on condition of anonymity, one official said, "Seven of the rhinos died but it has not been established as to why."
The seven were among 14 rhinos that were moved from Nairobi and Lake Nakuru national parks two weeks ago.
The programme was conducted jointly between KWS and the World Wide Fund (WWF).
“Translocating wild animals of this size is extremely challenging and not without risk, but black rhinos are under enormous threat so efforts to try and better protect them, such as translocation, are crucial for future generations,” WWF said in a statement on Friday.
The relocation of endangered animals, known as translocation, involves putting them to sleep for the journey and then reviving them in a process which carries risks.
But the loss of half of them is highly unusual.
The black rhinos were moved from Nairobi and Lake Nakuru national parks to Tsavo East last month in an operation trumpeted by Tourism Minister Najib Balala.
He is yet to comment on the tragic outcome.
Prominent Kenyan conservationist Paula Kahumbu said officials must take responsibility and explain what went wrong.
"Rhinos have died, we have to say it openly when it happens, not a week later or a month later," she said.
"Something must have gone wrong, and we want to know what it is."
Save the Rhinos estimates there are fewer than 5,500 black rhinos in the world, all of them in Africa, while Kenya's black rhino population stands at 750, according to the Worldwide Fund for Nature.
According to KWS figures, nine rhinos were killed in Kenya last year.
In May, three more were shot dead inside a specially-protected sanctuary in northern Kenya and their horns removed, while in March the last male northern white rhino on earth, an elderly bull named Sudan, was put down by Kenyan vets after falling ill.