The keepers of three of the last six northern white rhinos on Earth say it is highly unlikely the three will ever reproduce naturally.

Kenya's Ol Pejeta Conservancy announced Wednesday that recent medical examinations of the three show the species is doomed to extinction, unless science can help. A park official says in vitro fertilization and genetics-based science known as de-extinction are being examined.

Five years ago four northern white rhinos were flown from a zoo in Prague to Kenya where it was hoped that the natural environment could lead to a pregnancy. One of the two males died earlier this year of an unknown cause. The other three are effectively infertile.

Though the northern white rhino is nearly extinct, southern white rhinos and black rhinos are not.