The UN Human Rights Council in Geneva asked UN rights chief Zeid Ra'ad Al Hussein "to urgently organise and dispatch on the most expeditious basis possible a mission by independent existing experts."
At the start of the special council session on the spiralling violence in Burundi, Zeid had warned the country was "on the very cusp of civil war", insisting "the situation needs urgent, concerted, decisive attention from the international community."
The warnings of a risk of looming ethnic bloodshed came as the 54-member African Union said the continent "will not allow another genocide to take place on its soil," a reference to the horror in Rwanda in 1994.
Today's resolution, tabled by the United States and passed by consensus, expressed "deep concern about recent extrajudicial killings and attempted killings."
It called on the Burundian authorities "to allow or to conduct thorough and independent investigations of all such incidents, ensuring that all perpetrators of extrajudicial killings be brought to justice."
The experts should provide a full report to the council during next year's September session, it said.
Burundi's crisis began in April when President Pierre Nkurunziza announced his intention to run for a controversial third term, which he went on to win in July.
At least 400 people have died in violence and more than 220,000 have fled the country of around 11 million since the crisis began, according to UN figures.
Zeid however stressed that other sources said the death toll was "considerably higher," and alleged that a number of young men had been pulled from their homes and executed.
