Najin, one of the world's last two remaining female northern white rhinos, at the Ol Pejeta Conservancy in Laikipia county, northern Kenya. Picture: Lyu Shuai/Xinhua

London - They are the world’s most endangered animals, with just two females left.

But a world-first breakthrough in IVF could bring the northern white rhino back from the brink of extinction.

The last male, Sudan, died of old age in March, meaning scientists couldn’t save the species by getting animals to breed naturally. But now they have used frozen northern white rhino sperm to create embryos to implant into a female rhinoceros.

If successful, this could see a calf born within three years in Kenya – and even more at Longleat Safari Park in Wiltshire.

The embryos are not pure northern white rhinos, as they were created using the eggs of the closely related southern white rhino. But the same method could soon be used with eggs taken from the surviving northern rhinos in captivity in Africa. Better still, the embryos give researchers a source of stem cells, which could be turned into better quality sperm and eggs.

While IVF has been done in horses, it is the first time embryos have been created which have a chance of becoming rhino calves, by reaching the ‘blastocyst’ stage at which they are ready to be implanted.

Professor Thomas Hildebrandt, who led the experiment from the Leibniz Institute for Zoo and Wildlife Research in Germany, said: ‘These are the first in vitro-produced rhinoceros embryos ever. They have a very high chance to establish a pregnancy once implanted into a surrogate mother. Our results are solid, reproducible and very promising. Now we are well prepared to go to Kenya and collect oocytes from the last two northern white rhino females in order to produce pure blastocysts.’

Because of the animals’ size, IVF is trickier in a rhinoceros than a human being, requiring a 7ft instrument to harvest eggs.

The fertility treatment used frozen sperm from three dead northern white bulls.

The scientists, whose results are published in the journal Nature Communications, say they could put hybrid embryos into female rhinos in just over a year. A pregnancy lasts 16 months, so calves could be born by late 2020 or early 2021.

Alternatively, if scientists are allowed to take eggs from the two living northern white rhinos, a pure-bred calf could be born.