The Philippines assured the United States Tuesday it will honour its obligations as a military ally following volleys of profane tirades by unpredictable President Rodrigo Duterte.

After calling US leader Barack Obama a "son of a whore" last week, Duterte said he was "not a fan" of Washington and on Monday called for the small number of US military advisers to leave the southern Philippines.

On Tuesday Duterte also said Filipino forces would not participate in future joint patrols with the US in the South China Sea, where Manila claims waters that China insists are part of its territory.

But top officials moved to stem the damage, saying that it was business as usual between the Philippines and its powerful ally, whose backing is essential as it jousts with China over the maritime dispute.

"There is no shift in so far as our policy is concerned with respect to our close friendship with the Americans," Foreign Secretary Perfecto Yasay said Tuesday.

The president's spokesman also insisted that Duterte's comments were providing context to the conflict in the south, not a reversal of policy.

"These were not directives to leave, OK? But this was a context on why we have a conflict (in Mindanao). In other words, he's giving a broad historical, cultural landscape," Duterte's spokesman Ernesto Abella said.

Yasay, who was flying to Washington later in the day for talks, added the Duterte administration would honour existing defence agreements including a 2014 accord giving the US military access to at least five Philippine bases.

One of the bases is located in the southern region of Mindanao, where the government is battling Islamic militants who have offered allegiance to Islamic State gunmen in the Middle East.

The Pentagon in June also deployed warplanes and about 120 personnel in the northern Philippines for short-term training missions aimed at ensuring the allies' access to the South China Sea.

Duterte's predecessor Benigno Aquino had pursued closer military ties with the United States as part of plans to improve the deterrence capability of his militarily weak nation.

In April, the Philippine Navy began joint South China Sea naval patrols with the US as the Pentagon responded to muscular Chinese actions in the sea, including building artificial islands over disputed reefs.

- Umbilical cord -

On Tuesday Duterte said the country would refrain from joining similar excerises to avoid ramping up tensions.

"We will not join any expedition of patrolling the seas. I will not allow it because I do not want my country to be involved in a hostile act," said the president.

Washington said on Monday that Manila had not officially communicated Duterte's demand to pull US military advisers, deployed in Mindanao for short periods to train troops battling Muslim extremists.

Yasay attempted to downplay Duterte's comments, saying Tuesday they were "in the context of wanting to save the lives of these Americans who might be exposing themselves to unnecessary risk" from militant attacks.

Duterte, 71, has said the row was triggered by State Department criticism of his controversial war on drug crime, which has left about 3,000 people dead since he began his six-year term on June 30.

Obama has said Duterte must conduct his crime war "the right way", protecting human rights.

Zachary Abuza, a Southeast Asian security expert at the National War College in the United States, said Duterte's actions towards the US were worrying in light of militant activity in Mindanao.

"It is going to take a lot of work to get this relationship back on track," Abuza told AFP.