The Philippines and the United States launch war games on Tuesday against the unusual threat of American forces being ejected from the Southeast Asian nation as its firebrand leader pivots to China.

President Rodrigo Duterte has launched a sustained verbal assault on the United States, the Philippines' former colonial ruler and mutual defence partner, since he took office on June 30 in response to criticism of his deadly war on crime.

Duterte has in recent days warned the war games will be the last of his six-year term, and threatened to scrap a defence pact implemented by his predecessor that was meant to see more US troops in the Philippines to counter Chinese expansion in the South China Sea.

"Better think twice now because I will be asking you to leave the Philippines altogether," Duterte said on Sunday as he launched a tirade against the Americans full of typical invective.

"The Americans, I don't like them... they are reprimanding me in public. So I say: 'Screw you, fuck you'," he said, while signalling again that he wanted to forge closer alliances with China and Russia.

Last week Duterte, 71, also claimed the CIA was plotting to assassinate him.

This came after he branded Barack Obama a "son of a whore" in response to being told the US president planned to raise human rights concerns over his drug war.

Duterte has vowed to eradicate illegal drugs in the Philippines, warning the nation is in danger of becoming a narco-state.

His crime war has seen more than 3,000 people killed, with the United Nations, the European Union and rights groups raising concerns about alleged extrajudicial killings and a breakdown in the rule of law.

Duterte has insisted he is not doing anything illegal, yet at the same time said he would be "happy to slaughter" three million drug users.

He also likened his crime war to Nazi leader Adolf Hitler's effort to exterminate Jews, but following an outcry apologised for referring to the Holocaust.

- China circling -

Two thousand troops from both sides will take part in the war games, including in waters close to flashpoint areas of the South China Sea.

China claims nearly all of the sea, even waters close to the Philippines and other Southeast Asian nations, and has in recent years built artificial islands in the disputed areas capable of hosting military bases.

To counter China, the Philippines' previous president, Benigno Aquino, sought to draw the United States closer. This included the signing of the Enhanced Defence Cooperation Agreement that Duterte now wants to scrap.

Duterte appears intent on adopting the opposite tactic, saying recently he hopes to travel to China and meet with President Xi Jinping.

Aquino also launched international legal action that in July saw a UN-backed tribunal declare China's vast claims in the sea illegal.

However Duterte has declined to use the verdict to pressure China.

He has also said there will be no joint patrols with the United States in the sea.

Still Duterte's comments have not filtered down into government policy, and it remains unclear how badly he is prepared to damage ties with the United States.

"The (US military) relationship has not changed as of today," Philippine defence department spokesman Arsenio Andolong told AFP.

The US Embassy in Manila also said Philippine authorities had not officially communicated Duterte's statements.

"We continue to focus on our relationship with the Philippines and we're going to continue to work together in many areas of mutual interest," embassy spokeswoman Molly Koscina told AFP.