"The war on drugs does not work," Agnes Callamard, the UN's special rapporteur on extrajudicial killings, told a human rights conference in Manila.

"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, as has been well documented around the world," she said.

The comments came after US President Donald Trump provoked an avalanche of criticism for inviting Mr Duterte, a foul-mouthed firebrand former provincial mayor, to the White House, and praised his efforts to rid the country of drugs.

Two US legislators on Friday introduced legislation in the US Senate seeking to restrict the exportation of arms by the US to the Philippines, as the death toll under the crackdown almost reaches 8000, including children as young as five.

Republican Senator Marco Rubio, chairman of the US foreign relations sub-committee, said the war on drugs is "deeply alarming".

"This is not the right way to conduct an anti-drug campaign, and our legislation reflects our sincere desire to work with the Philippines to support human rights, expose narcotics networks emanating from mainland China and other countries, and use a public health approach to responsibly counter the danger drugs pose to our society," he said.

Mr Duterte's officials scoffed at the bill which would require the US State Department to report on the human rights situation in the Philippines before it could be passed.

"We have plenty of arms," said the President's chief legal counsel Salvador Panelo.

In his latest comments, Mr Duterte declared it will take him three years to stabilise the Philippines which has one of Asia's highest rates of drug use.

He promised to step-up the drugs war and efforts to curb criminality and corruption.

Ms Callamard's comments angered Mr Duterte, whose spokesman Ernesto Abella said she had failed to notify the government of her visit to Manila in a "clear signal" she was not interested in an objective view.

But Ms Callamard said she had informed Manila of her trip in advance and was participating in an academic human rights forum in her private capacity.

She praised Filipinos who have spoken out against the killings.

"I have followed the testimonies of the relatives of victims, I have seen the brave work of civil society actors, lawyers, human rights defenders, academics, senators," she said.

Mr Duterte was elected promising to wipe out illegal drugs within months, declaring among other outlandish statements that fish in Manila Bay would grow fat from the bodies of drug users and pushers.

A complaint has been filed in the International Criminal Court at The Hague citing Mr Duterte and 11 of his officials for crimes against humanity.

The court has yet to decide if it would accept the case.