By Ed Stoddard

JOHANNESBURG (Reuters) - South Africa's Kruger National Park is inviting bids for rhinos under a plan to move 500 of the animals to safety to counter a wave of poaching for their horns, highly prized in some Asian countries as a sign of wealth.

In newspaper ads on Sunday, South African National Parks invited potential buyers to "make a written offer to purchase white rhinos in batches of 20 or more".

Private ranchers own around 5,000 of South Africa's 20,000 rhinos, part of a thriving game farming industry in Africa's most advanced economy that caters to eco-toursim and hunting.

The government is now turning to the private sector because the Kruger has become the main poaching ground for the animals.

More than 1,000 were poached in South Africa last year, three times the tally in 2010, to meet soaring demand for rhino horn, coveted as an ingredient in traditional medicine in fast-growing economies such as China and Vietnam.

Government data shows at least 821 have been killed illegally so far this year, 531 of them in Kruger. The 2014 total is widely expected to exceed 2013, when a record number of rhinos were poached.

People buy them as an investment for a number of reasons. Rhinos are used to attract tourists to a lodge for game viewing and for legal trophy hunts and are traded in an open market. Some ranchers have also bought rhinos in the hope that trade in horn will be legalised at some point.

But given the rising risks in owning rhinos and the growing costs of keeping them secure, the planned sales might not get many takers.

"You are asking someone to put a large amount of money on the table in a speculative venture," Pelham Jones, chairman of the Private Rhino Owners Association, told Reuters.

Private rhino owners were already spending around 270 million rand ($24 million) a year just on security to protect their investment and the species, Jones said.

Rhino auction prices have not performed as well as other species such as cape buffalo, said Flippie Cloete, an economist at South Africa's North West University.

Between 1991 and 2013, the average price of white rhinos sold at auction increased to 277,351 rand from 180,247 rand - a rise of just 54 percent.

Cape buffalo prices over the same period leapt almost five-fold to just over 502,000 rand from around 118,000 rand.

The Kruger operation will see around 500 rhinos captured and hopefully sold to mostly private buyers - a massive logistical operation involving darting the animals from helicopters.

The first phase of the operation is under way, with some rhinos already moved to holding pens, South Africa National Parks spokesman Chris Vick said.

The focus is on Kruger's eastern border with Mozambique, one of the world's poorest countries, where villagers are lured by the promise of quick money into poaching.

(Editing by Stella Mapenzauswa and Alison Williams)