The Philippines said it would complain to the United Nations after one of its human rights investigators made an unannounced visit to Manila.
Agnes Callamard, the UN Special Rapporteur on extrajudicial killings, gave a speech at a policy forum at the University of the Philippines on Friday.
Ms Callamard has long been vocal about allegations of summary executions as part of the war on drugs.
She wanted to pay an official research visit last year but said the government’s conditions — including that she took part in a public debate with the president — were unacceptable.
Presidential spokesman Ernesto Abella said: “We are disappointed that, in not contacting our government in advance of this visit, she has sent a clear signal that she is not interested in getting an objective perspective.
“Callamard has arrived in the Philippines in a manner that circumvents all recognised United Nations protocols for such visits.”
Speaking at the forum, Ms Callamard said the UN General Assembly had concluded last year that drug wars did more harm than good.
“Badly thought out, ill-conceived drug policies not only fail to address substantively drug dependency, drug-related criminality, and the drug trade, they add more problems,” she said.
She later told reporters that she had no hidden motive for her visit. “It’s only for the purpose of academic conference,” she said.
At the same forum, two British drug policy experts said that drug addiction in the Philippines was below the global average — suggesting the war on drugs was fuelled by a “slightly manufactured crisis”.
Benjamin Reyes, chairman of the London-based Dangerous Drugs Board, said the global rate stood at 5.2 per cent — compared to just 2.3 per cent in the Philippines.
He added that even if the much-disputed claim of four million Filipinos being addicts was correct, the country would still be below the global average.
Commenting on the figures, John Collins, director of the London School of Economics’ International Drug Policy Programme, said: “So there is an element where this does feel like a slightly manufactured crisis.
“What you need to look at is what was the necessity for the recent declaration of the war on drugs?
“And so there is a perception that this is at a crisis point and there has to be severe action.”
Asked if Duterte’s drug war was likely to fail, Mr Collins said: “Based on international experience, it has virtually never worked.”