President Rodrigo Duterte and philanthropist George Soros.

MANILA - President Rodrigo Duterte said on Friday his war on drugs had cut back the supply to "very low" levels and thanked China for supporting his crackdown, but swore repeatedly at ally the United States for criticizing it.

Duterte said his bloody campaign against drugs had successfully reduced the narcotics flow, but conceded there were signs that criminals had now turned to kidnapping, another problem he planned to tackle.

"There is a very low supply of drugs now. But, there is a shift to kidnapping by these idiots," he said during a televised speech. "This is a new game, so be careful. Give me time to talk to God."

The crime-busting former mayor of the once lawless Davao City said last week he had spoken to God and promised him he would no longer use bad language.

But his vow has not held long. On Friday, he got angry again at former colonial power Washington for its concerns about alleged summary killings and contrasted its stance with that of China, which has funded a huge drugs rehabilitation center.

"Now who helped? China," he said. "America, what did they say? 'Duterte, stop the extrajudicial killings. We hold you responsible'," he said.

"I said: 'You can go to hell. You're all shit. You look at us Filipinos like dogs... You're all really sons of bitches because you violated our dignity.'"

As a provincial outsider in May's presidential election, Duterte used his brashness and profanity to enhance his public appeal. Dubbed "the punisher" and "Duterte Harry", he was elected by a big margin.

That was aided by the promise of a drugs war, which has killed over 2,300 people in four months.

Duterte on Friday homed in on Human Rights Watch, which he said was attacking him to justify a $100 million, 10-year grant philanthropist George Soros promised it six years ago.

"This Human Rights Watch of New York, that belongs to Soros. Soros was the financier. That's him. It's his grant," he said.

"They have funding money. They will really attack to justify. They chose me... they're pounding on me. That is fine, editorials every day. I can swallow that."

Duterte's relentless assaults on Washington have baffled the country's biggest ally, but do not appear to have resonated among Filipinos and the local business community, which has expressed concern.

A recent opinion poll of 1,200 Filipinos showed they had far greater trust in the United States than they did in China, which Duterte has been praising and courting strongly.

Duterte invited his countrymen to protest if they disagreed with him.

"If you think America will be good for you, if you want to be a (U.S.) territory ... if it is to your personal interest, go ahead and join the demonstration," he said.

"And maybe you can convince me to leave the presidency. But at least I leave without being treated like a pig by the Americans."