Delegates at the "Preventing Genocide" conference in Stockholm this Wednesday were expected to issue the most powerful document ever condemning mass killing.
But US opposition to recognising ICC authority - widely considered the most promising new tools in the fight against genocide - will make any declaration less effective, according to delegates.
The final draft, to be immediately adopted through consensus by 50 countries, deals with all aspects of "ethnic cleansing" and goes further than the 1948 UN Convention on the Prevention and Punishment of the Crime of Genocide.
The conference secretary general, Krister Kumlin, told journalists that: "We're talking about a consensus document here, and that means everyone has to agree.
"Both the US and Europe agree that it's important to bring perpetrators to justice. The disagreement is about what tool to use.
"The conflict is obviously there. It's very sad, not least because it was the United States that once took the initiative to create the ICC," he added.
But Washington fears the court could become a forum for politically motivated prosecutions of US citizens, especially soldiers deployed abroad.
The White House has waged an international campaign to sign immunity from prosecution pacts with various countries after the World Court came into being in 2002.