In her first speech to the UN Human Rights Council, the former International Criminal Court judge said next April's already contentious UN conference on racism and xenophobia would be impoverished if the United States and others sat it out.
"Let's not forget that diversity of opinions is often an inherent and welcome characteristic of relationships among peers," she said. "Should differences be allowed to become pretexts for inaction, the hopes and aspirations of the many victims of intolerance would be dashed, perhaps irreparably."
The United States and Israel walked out of the last big UN summit on racism, held in Durban in 2001, saying it had become a forum for anti-Semitism.
Canada has said it will not take part in the follow-up meeting planned for Geneva, and the United States, Britain, the Netherlands, and France have said they may stay away if Israel's treatment of the Palestinians again stands to eclipse all else.
Some are also concerned that Islamic countries will try to use the conference to push a declaration that could stifle free expression by labelling criticism of religions as defamatory.