Officials in South Africa say that 57 rhinos have been killed by poachers across the country so far this year.

The Department of Environmental Affairs said Thursday that recent floods in Kruger National Park, thick vegetation and two weeks of bright moonlight contributed to the high number of deaths.

The department said 18 suspected poachers have been arrested this year.

A record 668 rhinos were killed in South Africa in 2012, an increase of nearly 50 percent over the previous year. Demand is growing in Vietnam and elsewhere in Asia where rhino horn is believed to have medical benefits despite evidence to the contrary. The horn is made of keratin, a protein also found in human fingernails.

Elephant poaching — for the beasts' ivory tusks —is also rising across Africa.