A UN Human Rights Council expert on Palestine has posted on his blog an antisemitic cartoon of a dog wearing a kippah and urinating on a statue of Blind Justice.
American academic Richard Falk, who has written blogs in support of the flotillas to Gaza, last week published the cartoon by "Mahjoob", a pro-Palestinian cartoonist who has previously depicted Ehud Olmert slicing up a Palestinian baby.
The executive director of the Geneva-based UN Watch Hillel Neuer said: "Depicting Jews as dogs and bloodthirsty has a dark history."
The organisation has written to the UN's High Commissioner for Human Rights Navi Pillay to convey "grave concern".
The letter says: "The cartoon is manifestly antisemitic, and, before a worldwide internet audience, incites hatred against Jews as well as against Americans."
Mr Falk has also used his blog to write about 9/11 conspiracy theories and has been reprimanded by UN Secretary General Ban Ki Moon for "inflammatory rhetoric" and for insulting victims of the 2001 terrorist attack.
US ambassador to the UN Susan Rice has previously recommended that Mr Falk be removed from his position at the UN.
Mr Neuer said: "We know that the UN does condemn unacceptable views, even when they are made in a personal capacity, so Navi Pillay should not hesitate to condemn this blog post."
Ms Pillay and the UN Human Rights Council could not be reached for comment.