A Sumatran rhino gave birth to a female calf at a sanctuary in Indonesia on Thursday, taking the critically endangered species a step further away from extinction.

The baby was born at 5:40 am on western Sumatra island, and within hours was walking around and feeding from its mother, authorities said.

It was the second baby born to rhino Ratu. Her previous birth four years ago marked the first time a Sumatran rhino had been born in an Asian breeding facility for more than 140 years.

The new calf and Ratu, whose name means "Queen" in Indonesian, were both in good health although the mother looked "exhausted", the government said.

"We are very thankful for this birth, as Sumatran rhinos are rare animals," environment ministry spokesman Novrizal Tahar told AFP.

Ratu was observed stretching in her maternity pen in recent days, a signal her long-anticipated delivery was nearing.

The birth took around two hours. Just two hours after being born, the calf -- which has not yet been named -- began walking and feeding, according a statement from the forestry ministry.

The birth "demonstrates the government of Indonesia's commitment, in cooperation with the Indonesian Rhino Foundation, towards rhino conservation efforts in Indonesia," it added.

- Under threat -

Sumatran rhinos are extremely rare, with just 100 believed to exist in the world. The birth is a major boon for the species, which last year was declared extinct in Malaysia.

Ratu, a wild rhino who wandered out of the rainforest and into the Sumatran Rhino Sanctuary in Way Kambas National Park a decade ago, had become pregnant after meeting with Andalas, a male rhino at the park.

Ratu's first baby, Andatu, was born at the sanctuary in 2012.

Births of Sumatran rhinos in captivity are rare. Thursday's birth was only the fifth of a Sumatran rhino in a breeding facility.

Despite being the smallest of the five remaining rhino species, Sumatran rhinos have very long pregnancies that last about 16 months.

Harapan -- the brother of Andalas -- was transferred from the United States to the Sumatran sanctuary last November in the hope he would find a mate.

In March, environmentalists made physical contact with a Sumatran rhino on the Indonesian part of Borneo island for the first time in 40 years, but it died a month later.

Covered in woolly hair ranging from reddish brown to black in colour, Sumatran rhinos are the only Asian rhinoceroses with two horns.

While Javan rhinos are considered the world's rarest, Sumatran rhinos are under increasing threat.

They are targeted by poachers as their horns and other body parts fetch high prices on the black market for use in traditional Chinese medicine.

In addition, their rainforest habitat on Sumatra island is being destroyed due to the rapid expansion of palm oil and pulp and paper plantations.