In a desperate bid to preserve a critically endangered species, a US zoo is taking the controversial step of trying to mate brother and sister captive Sumatran rhinoceroses.

The coupling of six-year-old Harapan and his older sister Suci could take place as early as August at the Cincinnati Zoo in Ohio, animal keepers say.

The tactic has stirred strong emotions, but Terri Roth, director of the Lindner Center for Conservation and Research of Endangered Wildlife, said she has no other choice.

"We are in a really tough spot and we just don't have any other options," she told AFP.

Conservation experts say there are as few as 100 Sumatran rhinos left in the wild in their native lands of Indonesia and Malaysia.

In captivity, there are only 10 in the world. Four of them are closely related, with three having been bred in recent years at the Cincinnati Zoo.

A fourth born in Indonesia last year was the son of a Cincinnati-bred captive rhino and a formerly wild rhino.

For Suci, who is marking her ninth birthday next week, the only available suitors are her relatives, Roth said.

The other captive male of age in Indonesia is her older brother.

Harapan, who is on the cusp of adulthood, was brought to Ohio earlier this month from his previous home at the Los Angeles Zoo.

Artificial insemination has never worked in Sumatran rhinos, so shipping sperm from a genetically unique mate is not a viable alternative.

"Unless Indonesia were to capture more animals and an unrelated male became available to us, we don't have the genetic diversity that we need," Roth said.

In the meantime, experts say female rhinos need to breed in order to keep their reproductive organs from developing cysts that eventually render them infertile.

"Female rhinos, if they don't reproduce they tend to lose the ability to reproduce. It is kind of use it or lose it with them," said Susie Ellis, executive director of the International Rhino Foundation.

Ellis said the population of Sumatran rhinos has dwindled dramatically due to humans encroaching on their land and poaching for their horns, which some Asian cultures believe carry healing powers though they consist of simple keratin, the same protein as in nails, hair and hooves.

"The species has really hit a crisis point," said Ellis.

"There are maybe as few as 100 animals left in the wild."

Still, the decision to breed siblings carries risks, including abnormalities, harmful genetic mutations and poor sperm quality in the offspring.

"In general, we are very much against the idea of breeding relatives because we know that more often than not it causes problems," said David Wildt, head of the Center for Species Survival at the Smithsonian Conservation Biology Institute.

"I don't know if I would call it a solution. I think it is a strategy. And it really opens up some healthy debate," he added.

"If those were the last animals on the planet, what else would you do? But this isn't the situation here, though. These aren't the last two animals on the planet."

Zookeepers are hopeful that the Indonesian government will take steps to capture some wild rhinos that may be in isolation already due to deforestation, and add them to their captive sanctuary population in order to boost breeding options.

Roth said she was aware of the publicity the decision to mate siblings would bring -- both good and bad -- and she hopes it raises people's interest in preserving the habitat of these precious creatures.

The Sumatran rhino "is not like the giant panda that everybody knows about and follows. I thought people need to know about this," she said.

"We are about to lose this rhino quietly, without a whimper, and I don't want to see that happen."