Poachers have shot dead three critically endangered black rhinos in a specially-protected sanctuary in northern Kenya, the wildlife service said on Thursday.Two adults and a calf were killed inside the 83 square kilometre fenced rhino sanctuary in Meru National Park and had their horns removed."It is with a heavy heart that we announce the poaching of three rhinos last night... at the Rhino Sanctuary in Meru N Park, where we lost two black rhinos & a calf," Kenya Wildlife Service (KWS) said in a statement on Twitter."Gun shots were heard and ambushes laid at strategic points till morning," it said, adding that this "did not yield results."KWS said "aerial and ground" searches on Thursday revealed the three carcasses "with the horns missing"."No sign of the gang was reported, but our teams, both covert and overt, are in hot pursuit with good leads," it said.Rhino horn is made of keratin - the same substance as human hair and fingernails - yet is highly sought after in Asia, where consumers believe its ground form can cure hangovers or be used as an aphrodisiac.Rhino and elephant poaching has reduced in Kenya in recent years, with nine rhinos and 69 elephants killed last year, according to KWS.But the country remains an important conduit for illegal wildlife products smuggled out of the port at Mombasa and Nairobi airport.Save the Rhinos estimates there are less than 5 500 black rhinos in the world, all of them in Africa, while Kenya's black rhino population stands at 750, according to the Worldwide Fund for Nature.Kenya is also home to the only two known remaining northern white rhinos on earth, after the last elderly male was euthanised in March.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.