Ex-Philippine leader Fidel Ramos has resigned as special envoy to China after criticising President Rodrigo Duterte's tirades against the United States and deadly war on crime.

Duterte said late Tuesday he had received a resignation letter from Ramos, an elderly stalwart of Philippine politics who had been a trusted ally of the new leader but recently branded his rule as a "huge disappointment and let-down".

According to the president, the pair had disagreed on US relations and the domestic crime war, which has claimed more than 4,000 lives in four months.

"I know he is pro-Western. He is a military man who studied there (in the US). You must remember Ramos finished his (studies) in West Point. He really does not want to fight (with the US)," Duterte told reporters.

Duterte appointed Ramos, president from 1992 to 1998, as his envoy to mend relations with Beijing, which had soured under the previous administration over competing territorial claims to the South China Sea.

Ramos, 88, travelled to Hong Kong in August on an "icebreaker" trip to meet with senior Chinese officials.

But Ramos did not join Duterte in Beijing last month for a state visit that cemented better ties and secured billions of dollars in Chinese investments and loans.

Ramos told GMA7 television this week he had resigned because he had done what was needed to improve ties with China.

But Duterte's comments pointed to a much deeper falling out with Ramos, who he had repeatedly publicly thanked for supporting his bid to become president.

Ramos, a former police and military general, has for decades been a powerful political force in the Philippines, and as armed forces vice chief of staff, he was key in turning the military against dictator Ferdinand Marcos during the 1986 "People Power" revolution.

Ramos has in recent weeks been openly critical of Duterte's attacks on the US and threats to end the two nations' military alliance.

Duterte's tirades have been partly in response to American criticism of his war on crime.

"So, what gives? Are we throwing away decades of military partnership, tactical proficiency, compatible weaponry, predictable logistics and soldier-to-soldier camaraderie just like that," Ramos wrote in a newspaper column to mark Duterte's 100th day in office.

Ramos has also been one of the few prominent political figures to directly criticise Duterte's anti-drugs crackdown.