Spokesman Robert Wood said Saturday that despite improvements from an earlier draft, the changes in the final text do not address U.S. concerns about anti-Israel and anti-Western bias. The administration had lobbied hard for more revisions so that it could participate.
Israel and Canada have already said they will not attend over concerns about a possible repeat of verbal attacks on the Jewish state.
The five-day meeting, being held in Geneva starting Monday, is intended to evaluate progress toward goals set by the first such conference which met in 2001.
That meeting, which ended four days before 9/11, was dominated by quarrels over the Middle East and the legacy of slavery. The United States and Israel walked out midway through the conference over a draft resolution that singled out Israel for criticism and which likened Zionism - the movement to establish a Jewish state in the Holy Land - to racism.
Those references were removed from the final declaration, though it did cite "the plight of the Palestinians" as an issue.
Many of the same issues - such as criticism of Israel - are now re-emerging in this latest meeting of the World Conference Against Racism, Racial Discrimination, Xenophobia and Related Intolerance.
The European Union had not decided Friday whether to attend the meeting or boycott it over Islamic nations' demands to condemn Israel and call for a ban on defaming religion.
"We have made clear ... that we cannot tolerate it if this anti-racism conference is turned into an accusatory event, a one-sided event against the state of Israel," said Thomas Steg, a spokesman for German Chancellor Angela Merkel.
Iran's President Mahmoud Ahmadinejad - who has repeatedly called for the destruction of Israel and denied the Holocaust - plans to speak Monday as the conference opens.
Some sticking points remained Friday that could unravel the conference, such as Iran's objection to a paragraph stating that the Holocaust must never be forgotten.
Direct references to Israel and to defamation of religion have been dropped from the draft document for this year's conference, but there is pressure from Muslim countries to reinsert them.
Many Muslim nations want curbs to free speech to prevent insults to Islam they say have proliferated since the Sept. 11, 2001 attacks on the United States. Riots erupted across the Muslim world after a Danish newspaper published cartoons of the Prophet Muhammad in 2005.
Last week the United States said it remained concerned about "restrictions on freedom of expression that could result from some of the document's language related to 'incitement' to religious hatred."
"There are still issues which remain and these are being discussed," said U.N. spokesman Ramu Damodaran.
U.N. Secretary-General Ban Ki-moon and the head of the Organization of The Islamic Conference, Ekmeleddin Ihsanoglu, will take part in the meeting's opening. Officials from 103 states have confirmed their participation, according to the U.N.
Jewish and Muslim lobby groups, as well as human rights groups, are prepared to turn out en masse. More than 2,500 participants of non-governmental organizations (NGOs) were expected to be observers, the U.N. said, though only a half-dozen NGOs are allowed to speak at the conference itself.
Nobel Peace laureate Elie Wiesel is among the many prominent defenders of Israel who will be present.
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