(Adds quotes from activist)

KUALA LUMPUR, July 5 (Reuters) - Low sperm counts and other reproductive problems are preventing pregnancy among Malaysia's endangered rhinos, a worrying trend that wildlife experts say could hasten its extinction.

Experts meeting on Borneo island this week to discuss ways to save the Borneo rhino said a major threat -- besides poaching -- was the animal's inability to reproduce.

"Maybe because they live in fragmented locations deep in the jungles and because of that, they rarely get the opportunity to mate," the New Straits Times newspaper on Thursday quoted Sabah Wildlife Department deputy chief, Laurentius Ambu, as saying.

But scientists also found that male rhinos suffer from low sperm count while many of their female counterparts have cysts in their reproductive organs.

"It's a mystery," he said. "We are curious to learn more."

The authorities attempts to encourage captive breeding had failed, Ambu said. "We will try our best to allow the rhinos to breed naturally," he added.

Conservation group, SOS Rhino, said some female rhinos held in captivity had developed tumours in their uterus, thus preventing pregnancy.

"It's more of a psychological disease due to imbalances of hormones or stress," its president Nan Schaffer told Reuters.

"It certainly has interfered with the reproduction of the animals in captivity," Schaffer, who is an expert on reproductive psychology, said by telephone from Sabah.

The wildlife department says there are between 30 and 50 rhinos left in the dense jungles of Malaysia's Sabah state, on Borneo. The animals are so secretive that the first photograph of one was only taken last year.

In April, global conservation group WWF said it had filmed the animal for the first time.

Scientists consider the Borneo rhino to be a subspecies of the Sumatran rhino.

Rhino horns, made of hair-like keratin fibres, have reputed aphrodisiac qualities and are a prized ingredient of traditional Asian medicine.

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