Four out of 6 endangered black rhinos that were relocated to Chad from South Africa have died, possibly of starvation, the two countries and an NGO said.

The six rhinos were transferred from South Africa in May, re-introducing the species into Chad for the first time since it died out in 1972, mostly owing to rampant poaching. Chad reported that two of them had died last month.

"Low fat reserves suggest that maladaptation by the rhinos to their new environment is the likely underlying cause," said the two nations and conservation group African Parks, which oversaw moving the rhinos to Chad's Zakouma National Park.

"The remaining two animals are being recaptured and placed in holding facilities in order to facilitate closer management."

Two decades ago, poaching had driven black rhinos to near extinction across their vast habitat in southern, eastern and central Africa. Their numbers fell by 98 percent between 1960 and 1998, but they have doubled since then to about 5,400 as conservation efforts grew.

None of the four that died in Zakouma had been poached. Tests were being run to establish the exact cause of death.

The relocation of the rhinos from South Africa was intended to help safeguard the long-term future of the species by spreading it out into more places.

However, a similar attempt to move black rhinos in Kenya also ended with animals dying. Eleven rhinos were moved into Kenya's Tsavo East National Park from other parks in June, but 10 of them died, apparently from drinking salty water. The 11th was attacked by a lion and died from his wounds.