South African authorities revealed Tuesday that a phosphate mine spillage has caused "extensive pollution" to a river in the country's famed Kruger National Park. Park officials said "highly acidic water" from a dam at the Palaborwa-based Bosveld phosphate mine spilled into a tributary of the Olifants River, killing thousands of fish. "It's extensive pollution given the number of fish we have witnessed floating over a 15 kilometer (nine mile) stretch of the river," the park's spokesman Ike Phaahla told Agence France Presse. "We haven't seen any of the big animals affected -- your hippos or your crocodiles," said Phaahla after a preliminary investigation. But the park's water resources manager Eddie Riddell estimated the number of dead fish at several thousand, pointing to major environmental damage. A fisherman notified the park in late December of a number of dead fish floating on the river, prompting the probe. Heavy rains that recently pounded the area are believed to have contributed to the spill. Investigations are focusing on establishing which chemicals flowed into the river and mapping the extent of the ecological damage. "We are still busy taking samples and assessing the impact but we do believe there is a major environmental damage," the general manager of the park's scientific research services unit Stefanie Freita-Ronaldson told AFP. The Kruger National Park is a vast reserve that is around two million hectares, four times the size of Yellow Stone park in Wyoming. Its lion, rhino and other "big-five" animals draw tourists from around the world. The pollution was first discovered during the peak festive season when the park's camps are bustling with holiday-makers. Officials said water supplies to camps were switched to borehole water as soon as the spill was suspected. "So there is absolutely no human health issue for tourists in the Kruger National Park, they are not using water from the Olifants at all," said Freita-Ronaldson. Water and wildlife officials held meetings with mine management Tuesday to work out ways to remedy the problem and prevent any future disasters.