The United Nations Human Rights Council voted on Thursday to set up an investigation into mass killings during Philippine President Rodrigo Duterte's self-described 'war on drugs' — a step that activists said was long overdue.

Key points: Filipino President Rodrigo Duterte has led a brutal policy against drug users and dealers

Filipino President Rodrigo Duterte has led a brutal policy against drug users and dealers Estimates of those killed range from the thousands to tens of thousands

Estimates of those killed range from the thousands to tens of thousands The UN is seeking access to the country to investigate and deliver its findings by 2020

Mr Duterte's government says that around 6,600 people have been killed by police in shootouts with suspected drug dealers since he was elected in 2016 on a platform of crushing crime.

Activists say the death toll is at least 27,000.

The first-ever resolution on Mr Duterte's policy, led by Iceland, was adopted by a vote of 18 countries in favour and 14 against, including China, with 15 abstentions, including Japan.

"This is not just a step towards paying justice for the thousands of families of victims of extrajudicial killings in the Philippines, but it is also a message that we collectively send out to those who have praised President Duterte," said Ellecer Carlos of the Manila-based rights group iDefend.

"This war on drugs, as we have repeatedly said, it's a sham war," he told a news briefing in Geneva.

Filipino activists say tens of thousands are being killed as police terrorise poor communities, using arbitrary drug "watch lists" to identify suspected users or dealers, and executing many of them under the guise of sting operations.

Activists say the poor are overwhelmingly caught up in the drug war. ( Supplied: Vincent Go )

Myca Ulpina, a 3-year-old killed on June 29 near Manila, was among the latest and youngest known victims of the crackdown.

Police say her father Renato had used his daughter as a human shield.

The Philippines' UN delegation lobbied hard against the resolution, which asked national authorities to prevent extrajudicial killings and cooperate with UN human rights boss Michelle Bachelet, who is to report her findings in June 2020.

Families of the drug war's alleged victims have called on the UN for help. ( AP: Bullit Marquez )

Manila promises 'far-reaching consequences'

Philippines Ambassador Evan Garcia, speaking after the vote, read a statement by his foreign ministry rejecting the resolution as "politically-partisan and one-sided".

His country is among the council's 47 members.

Mr Garcia said the Duterte administration was committed to upholding justice, adding:

"We will not tolerate any form of disrespect or acts of bad faith. There will be consequences, far-reaching consequences."

Laila Matar of Human Rights Watch criticised his comments.

"It was quite clear that they threatened consequences for those who had supported the resolution, which in turn makes us concerned for the many human rights defenders, civil society activists and journalists on the ground," she told the briefing.

Mr Duterte, asked by reporters in Manila whether he would allow access to UN rights officials to investigate, said: "Let them state their purpose and I will review it."

Human rights observers have raised concerns about the impunity of Mr Duterte's policy. ( Supplied: Vincent Go )

Reuters