GENEVA (Reuters) - Albinos whose body parts are prized by witch doctors are being attacked in rising numbers in Tanzania, with children who are still alive having their limbs chopped off, the top U.N. human rights official said on Tuesday.

"I strongly condemn these vicious killings and attacks, which were committed in particularly horrifying circumstances, and which have involved dismembering people, including children, while they are still alive," Navi Pillay, the U.N. High Commissioner for Human Rights, said in a statement.

The killing and mutilation of people with albinism - a congenital disorder caused by the absence of pigment in the skin, hair and eyes - is often linked to witchcraft, the statement said.

Some practitioners seem to believe the witchcraft is more powerful if the victim screams during the amputation, which is why the body parts are often hacked from live victims, it said.

Prosecutions are rarely successful, with only five known cases of convictions out of the 72 murders of people with albinism documented in Tanzania since 2000, it said.

"These crimes are abhorrent," Pillay said.

Four attacks targeting people with albinism, including three children, have been documented in Tanzania from January 31-February 15, she said. The crimes also reflected underlying discrimination.

Lugolola Bunzari, a 7-year-old boy with albinism, was brutally murdered in Kanunge village in Tabora region on January 31, according to Pillay, a South African lawyer who is also a former judge of the International Criminal Court.

"His attackers slashed his forehead, right arm and left shoulder and chopped off his left arm just above the elbow," she said, adding that the boy's 95-year-old grandfather also was killed as he tried to protect the boy.

Maria Chambanenge, a 39-year-old female albino, was attacked on February 11 by five armed men in Mkowe village in Rukwa region, who were later arrested, she said.

"They hacked off her left arm while she was sleeping with two of her four children," Pillay said.

"The Tanzanian authorities have the primary responsibility to protect people with albinism, and to fight against impunity, which is a key component for prevention and deterrence of the crimes targeting this exceptionally vulnerable community," she added.

(Reporting by Stephanie Nebehay; Editing by Michael Roddy)