The UN Human Rights Council was due to vote Thursday on a resolution mandating a "comprehensive" international review of the Philippines drug war, which watchdogs say has killed more than 20,000 people.
The resolution has faced strong pushback from President Rodrigo Duterte's government, which counters that the crackdown on narcotics has left just 5,300 people dead while retaining the strong support of many Filipinos.
The text to be voted on in the 47-member rights council requires the UN High Commissioner for Human Rights, Michelle Bachelet, to conduct "a comprehensive written report on the situation of human rights in the Philippines," over the coming year.
Filipinos deeply conflicted on Duterte's drug war
Activists said they had initially hoped the resolution would call for a formal "inquiry," but compromised on a "report" to raise the prospects of winning a majority.
Duterte called the former UN rights chief, Zeid Ra'ad Al Hussein, an "idiot" and a "son of a bitch" after Zeid raised concern about the drug war, which has reportedly included nightly slaying of suspects by police and masked gunmen.
Last week, Duterte spokesman Salvador Panelo described the resolution as the "latest demonisation attempt against the president" and said Manilla expected it "to fail."
Amnesty International said Thursday's vote in Geneva was "critical."
"The reality is that (the) Philippines government has sealed off all avenues for domestic accountability," Amnesty's regional director for east and southeast Asia, Nicholas Bequelin, said in a statement.
"This resolution would mark a vital step towards accountability and justice for victims of the government's campaign of extrajudicial executions and other human rights violations.
"Member states with a vote at the Council must support it... Their very credibility is at stake," he added.
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In addition to a report by Bachelet, the resolution proposed by Iceland and now supported by many EU states and Canada, also expresses concern about a range of alleged abuses in the Philippines, including extrajudicial killings and enforced disappearances.
The drug war launched in 2016 is Duterte's signature initiative and he has often reacted with fury when outsiders raise concerns about the project.
Earlier this month, he said "extrajudicial killing is okay, but not corruption", though he did not elaborate further.