The General Assembly adopted a treaty Wednesday that would ban nations from abducting perceived enemies and hiding them in secret prisons or killing them.
The United Nations received reports of about 535 disappearances last year, many of them in Colombia, Nepal and the breakaway Russian republic of Chechnya.
The treaty has been under negotiation since 1992. The measure was approved by consensus and will come into force 30 days after 20 countries ratify it.
The United States, accused of transferring terrorism suspects to secret jails in other countries, did not address the assembly, but rights experts said they did not expect Washington to ratify the pact.
The measure offers a first definition of disappearance in international law, calling it detention, abduction or deprivation of liberty by state agents followed by a refusal to acknowledge deprivation and a placing of the disappeared outside legal protection.
It requires governments to outlaw secret detention and undeclared facilities, and establishes the right of families to learn the fate of relatives and to seek reparations.