Some 15,000 crocodiles escaped from a South African reptile farm in flood waters this week and were on the loose in and around one of southern Africa's biggest rivers, a newspaper reported.

"There used to be only a few crocodiles in the Limpopo River. Now there are a lot," Zane Langman, whose in-laws own the farm in the northern part of the country told Beeld newspaper.

Langman said only half the escaped crocodiles from the Rakwena Crocodile Farm close to the Botswana border had been recaptured, the report said.

Langman added that farm gates were opened out of fear the rushing flood water would crush the crocodiles.

Officials from the farm were not immediately available for comment.

One crocodile apparently from the farm was captured about 120 km (75 miles) away at a school rugby field, it said.

Heavy rains and flooding have claimed at least 20 lives in Mozambique and South Africa and led to the evacuations of thousands.