Philippine President Rodrigo Duterte said Saturday he would not sever his nation's alliance with the United States, as he clarified his announcement that he planned to "separate".

"It's not severance of ties. Severance is to cut diplomatic relations. I can not do that. Why? It's in the best interests of my country that I don't do that," Duterte told reporters in his hometown of Davao after returning from China.

The firebrand leader signalled on Thursday during his four-day state visit to Beijing that he intended to end the Philippines' 70-year alliance with the United States in favour of China and Russia.

"I announce my separation from the United States," Duterte told a group of Chinese businessmen.

"America has lost. I've realigned myself in your ideological flow and maybe I will also go to Russia to talk to (President Vladimir) Putin and tell him that there are three of us against the world: China, Philippines and Russia. It's the only way."

Until Duterte took office on June 30, the Philippines had been one of the United States' most important and loyal allies in Asia, and a key to President Barack Obama's "pivot" to the region.

But since becoming president Duterte has done a dramatic foreign policy U-turn that has baffled Washington.

US State Department spokesman John Kirby said on Thursday that the United States would seek clarification from the Philippines about the "separation" remark.

"It's not clear to us exactly what that means in all its ramifications," he said.

Duterte on Saturday gave a series of comments to clarify those remarks.

"Sever is to cut. Separate is just to chart another way of doing," he said.

"What I'm really saying was separation of foreign policy, which in the past and until I became president, we always followed what the United States would give the cue."

- Another tirade -

Nevertheless, Duterte launched another tirade against the United States for criticising his war on crime, which has left more than 3,600 people dead and raised fears about extrajudicial killings.

Duterte said a defence pact signed in 2014, known by the acronym of EDCA and which allows for a much greater US military presence in the Philippines, remained in jeopardy.

"It will affect EDCA and the rest of the agreements, maybe, I will have to consult the military, the police and everybody," he said.

Duterte also said he did not care if the United States or the European Union cut their foreign aid to the Philippines, worth hundreds of millions of dollars a year, over concerns about human rights abuses in his war on crime.

"Assistance, USAID, you can go to hell," he said, referring to the US government's overseas economic and development assistance organisation.

Duterte often laces his rhetoric with vulgar language, and has repeatedly referred to US President Barack Obama as a "son of a whore".

On Saturday he let loose again at US and European critics of his war on crime.

"You sons of whores. Your euro, that's a piece of paper. You run out of toilet paper, you wipe up your ass," he said in a rant that at times appeared not to make sense.

"You guys are bullshit. Why am I saying this? It sounds the height of vulgarity. You started it."

Duterte, 71, also repeated criticism of the American invasion of Iraq in 2003, saying it was based on a lie and triggered turmoil in the Middle East that cost many lives.

"If there is one thing that America has failed miserably, it is in the province of the human dignity," he said at the end of his critique of US foreign policy in the Middle East.

Duterte said he also endorsed Russian efforts to keep Syrian President Bashar al-Assad in power.

"If Assad is out they (the United States) will have destroyed the entire Middle East," he said.