Two white rhinos in the Free State were apparently drugged and robbed of their horns at the Willem Pretorius nature reserve near Ventersburg, environmental authorities said on Thursday.

Free State environmental affairs spokesperson on law enforcement, Werner Boing, said it was suspected the two adult white rhino cows were darted and drugged over the weekend.

A power-saw or grinder was used to remove the horns.

Officials said one of the reserve's rhino watchers reported on Wednesday that one of the pregnant cows had no horn.

Conservation authorities organised a chopper pilot and a veterinarian to find the rhino to inspect her.

They also found another cow, with a calf, without a horn.

The animals were drugged to treat the wounds and to take samples for a criminal investigation.

Boing said while the animals were not killed, the situation was tragic.

He said the pregnant cow had been kicked out of the herd, while the other cow cannot protect her calf.

Boing said both animals had been put under extreme stress through being drugged twice within a couple of days.

He urged the public to come forward with information about the incident.