Two of the six critically endangered black rhinos that were relocated in May from South Africa to Zakouma National Park in Chad have died.

Wildlife charity African Parks said the male and female rhinos were not poached and whilst the exact cause was not yet known, they may have died as a result of a virus.

A vet is carrying out a postmortem to find out the exact cause of death, the charity added.

African Parks confirmed that the four other rhinos are still alive and that they are being "closely monitored".

Before the relocation, black rhinos had not been seen in Chad since the early 1970s, largely as a result of rampant poaching.

Officials say the elephant population in Zakouma is on the rise after more than 90% of the animals were poached between 2002 and 2010.