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 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-kilometre (nine-mile) stretch of the river," park spokesman Ike Phaahla told AFP.

"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.

The mine owners will face prosecution for violating the country's water laws.

"We have already... laid criminal charges against Bosveld Phosphates for contravening the National Water Act," said Nigel Adams, a director in South Africa's department of water and environmental affairs.

Susceptible to diseases

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 Freitag-Ronaldson told AFP.

Scientists fear the impact could be "very serious" and long term.

"The sulphates completely inhibit the fishes' ability to uptake certain very vital vitamins and minerals and they die," said Koos Pretorius, a veterinary surgeon and environmental activist.

"And when they die, the crocodiles come and they gorge on them," get sick and die too. "So it's very serious," he said.

The poor uptake of essential minerals causes the animals to be "very susceptible to diseases" and to have very poor growth, reproduce less and weak babies that die easily.

"It's what we call an erosion disease. It's like HIV, it's an underlying problem ... some people are affected but many, many people are living with the consequences," said Pretorius.

The animals that are dying also release nutrients favourable for the growth of algae which itself can be toxic and causes less oxygen -- leading to more fish dying.

Some types of algae are highly poisonous and can kill fish instantly, he said.

"Blue algae is extremely carcinogenic and toxic. An animal can drink from the water and fall over and die," he warned.

Mass deaths from polluted waters are not new in the Kruger National Park.

In 2008, over 170 crocodiles died over a period of seven months, but experts struggled to pin down the precise cause.

The Kruger National Park is a vast reserve that is around two million hectares, four times the size of Yellowstone 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 Freitag-Ronaldson.

Water and wildlife officials held meetings with mine management Tuesday to work out ways to remedy the problem and prevent any future disasters.