Beijing on Friday expressed support for a bloody crackdown on illegal drugs in the Philippines overseen by President Rodrigo Duterte, ahead of a visit to China by the controversial leader.

The crackdown has left more than 3,700 people dead since July, both at the hands of police as well as in unexplained circumstances, according to official data, prompting condemnation from Western nations, the UN and the International Criminal Court, among others.

"We understand and support the Philippines' policies to combat drugs under the leadership of President Duterte," Chinese foreign ministry spokesman Geng Shuang told a regular briefing.

Duterte will visit China next week for his first foreign visit outside of Southeast Asia since assuming the presidency in June, highlighting his efforts to restore ties with Beijing strained by competing claims in the South China Sea.

Duterte has looked to build closer relationships with China and Russia while launching repeated tirades against the United States, the Philippines' defence ally and former colonial ruler.

His outspoken comments have been largely in response to US criticism of his war on crime, which has raised fears about extrajudicial killings.

Duterte has cancelled joint patrols with the United States in the South China Sea, said he may scrap a defence pact that allows thousands of US troops to rotate through the Philippines, and threatened to eventually cut ties completely.

He has also branded US President Barack Obama a "son of a whore" for expressing concern about human rights in the drug war.

In contrast, he has described Chinese leader Xi Jinping as "a great president", and praised China and Russia for showing respect in not criticising his crime crackdown.