Philippine President Rodrigo Duterte has hit out at "very stupid" United Nations criticism of his controversial war against illegal drugs that has claimed an estimated 1,000 lives, warning the global body not to interfere.

"Here comes the UN, easily swayed, and coming with a very stupid proposition," Duterte said in a speech on Wednesday at an event for police officers also attended by foreign diplomats.

"Why would the United Nations be so easily swayed into interfering in the affairs of this republic?"

Rodrigo Duterte: Shoot a drug dealer, get a medal

Duterte said that while the UN was quick to criticise his administration, the world body "is keeping silent" on the violence in the Middle East.

Referring to conflicts in the region, Duterte said he had not yet heard any public outcry targeted at "countries who are bombing villages and communities, killing everybody there, including the goats and the cows and the dogs".

UN Secretary-General Ban Ki-Moon and the UN Office on Drugs and Crime had earlier condemned Duterte's "apparent endorsement of extrajudicial killings" in his fight against illegal drugs and crime.

According to police figures, more than 600 suspected drug users, pushers and traffickers have been killed since July 1 - although some local media have put the death toll at 1,000.

'No longer viable as human beings'

But Duterte argued that the numbers were low compared with violence elsewhere in the world.

"What's the problem? You inject politics. Only one thousand died and you put my country in peril, in jeopardy," he said.

"For those who are killed by drug syndicates, we can only investigate. But do not attribute the acts of other criminals upon my government."

The 71-year-old president noted that millions in the Philippines are being "devastated by drugs" including drug addicts "no longer viable as human beings on this planet".

Duterte won the May election on a promise to wage a war on illegal drugs and other crime that would claim tens of thousands of lives.

He has ordered police not to hesitate to kill and even urged ordinary citizens and communist rebels to join in the war against drugs.

On Wednesday, at least 18 police officers were dismissed after testing positive of illegal drugs, according to the Manila-based news website, Rappler.

He has repeatedly scoffed at human rights groups opposed to the killings, saying, "I don't care about human rights".

He has also threatened to declare martial law if the judiciary interferes with his policies.