Poachers have killed record numbers of rhinos in South Africa in recent years, but some conservationists believe the toll could be even higher than the official figures.

South Africa's environment ministry, which reported 1,215 rhino poachings last year, said Wednesday that aerial surveillance and other "concerted efforts" are made to detect carcasses of killed rhinos.

The ministry says procedure requires the dead fetus of a pregnant rhino killed by poachers to be included in poaching statistics, but says "some" may be missed if scavenging animals reach the carcass before investigators.

Allison Thomson, founder of an anti-poaching group, says there are cases included in the official statistics. As an example, Thomson says lions or other predators probably kill some vulnerable rhino calves whose mothers are poached.