The General Assembly approved an agreement with Cambodia to establish a tribunal to try aging Khmer Rouge leaders for genocide and crimes against humanity.
Approval of the agreement Tuesday by the 191-member body now puts the onus on Cambodia's legislature to take similar action to create the UN-assisted court.
Last week, Cambodian Prime Minister Hun Sen said that after the draft was approved by the General Assembly, "Cambodia can start the procedures for ratification of this agreement."
Even with approval by the UN and Cambodia, it still could be a long time before any trial is held.
An estimated 1.7 million Cambodians died from starvation, disease, overwork and execution when the communist Khmer Rouge held power in 1975-79. The movement collapsed in 1998, but none of its leaders has ever been tried for the group's atrocities, and many still live freely in Cambodia.
The General Assembly resolution, which was adopted by consensus, calls for voluntary donations to fund the tribunal.
Japan's UN Ambassador Koichi Haraguchi, one of the co-sponsors of the resolution, said approval of the agreement brought justice for one of the worst crimes of the 20th century a step closer.
US representative Charles Twining said "the United States remains committed to the establishment of a credible tribunal and supports the substance of the current resolution." But he said, without elaborating, that the US would "disassociate itself" from the General Assembly's consensus because of concerns about the timing of its adoption.
Without elaborating, Twining said it would have been better to delay consideration of the text until after the elections for Cambodia's National Assembly in July. US officials had no further explanation.
The agreement between the UN and the Cambodian government was reached in March in Phnom Penh after more than five years of difficult negotiations.
Talks broke down in February last year when the UN concluded Cambodia was not willing to agree to terms that would ensure free and fair trials.
At the time, UN officials cited the Cambodian government's insistence that its national laws prevail, which they feared would reduce the world body's role to that of a mere provider of technical assistance.
UN legal counsel Hans Corell, who negotiated the agreement approved Tuesday, said in March it has standards and provisions that are "much, much stronger to protect the integrity" of the judicial process.
The UN originally wanted a majority of international judges, but Cambodia has insisted on a majority of Cambodian judges. The trial and appeals chambers will have a majority of Cambodian judges, but under the agreement, at least one international judge has to join in any judgment.
Human-rights groups and some legal experts, nonetheless, consider Cambodia's judicial system too corrupt and vulnerable to political pressure to ensure justice.
Amnesty International and other rights organizations have urged the UN not to approve the draft agreement in its present form, saying justice would not be served.