In Washington, Sean McCormack, a US state department spokesman, declined to comment except to say that the US helped draft the treaty, but that the final text "did not meet our expectations".
McCormack declined to comment on whether the US stance was influenced by the administration's policy of sending terrorism suspects to CIA-run prisons overseas, which George Bush, the US president, acknowledged in September.
'Deprivation of freedom'
The convention defines forced disappearances as the arrest, detention, kidnapping or "any other form of deprivation of freedom" by state agents or affiliates, followed by denials or cover-ups about the detention and location of the missing person.
Many delegates expressed hope that other nations would sign the treaty by the end of the year. Some European nations have expressed support for the treaty but face constitutional hurdles or require a full cabinet debate before signing, French and UN officials said.
Louise Arbour, the UN High Commissioner for Human Rights (UNHCR), called the treaty an important step both in preventing injustices common years ago and barring newer tactics that often fall through regulatory loopholes.
Arbour urged US officials to sign and ratify the treaty. She noted that America often backs activities of the UNHCR without formally signing up to them.
She called the treaty "a message to all modern-day authorities committed to the fight against terrorism" that some methods applied in the past are now "not acceptable, in a very explicit way".
Douste-Blazy said more than 51,000 people were victims of forced disappearances in more than 90 countries since 1980 and 41,000 of the cases remain unsolved.
He said: "Men and women disappear every day on every continent, for defending human rights, for just opposing their governments' policies or simply because they want justice.
"The situation could not continue to go unpunished. It required a strong response from the international community."