Philippine President Rodrigo Duterte says he is concerned that tensions between China and the United States could drag Manila into a conflict between the two world powers, local media reported.

Duterte was speaking on Wednesday on the sidelines of a summit in Singapore, where heads of state of the 10-member Association of South East Asian Nations (ASEAN) are meeting their Asia-Pacific counterparts.

The South China Sea dispute has remained a perennial thorn in the side of many of its member countries. China has claimed ownership of almost all of the maritime territory, building military installations and facilities even as the Philippines, alongside Vietnam, Malaysia, Taiwan and Brunei, have laid competing claims to it.

Duterte said while "everything has been excellent" between ASEAN and China with regard to managing the sea dispute, that could be put at risk given "friction between the Western nations and China," the Manila Bulletin reported.

Duterte pointed out that the mutual defence treaty his country has in place with Washington would require it to defend the US, and vice versa, in case of an attack on the other's territory.

"And [if] there's some — a serious miscalculation. I'd like to [tell] China — that is why at all costs, we must have the Code of Conduct (CoC)," the report quoted him as saying.

Duterte also appeared to accede to China's occupation of the disputed territory, even though ASEAN has consistently maintained that any agreement on the South China Sea would have to be arrived at via consensus from all competing countries.

"So you're there, you're in possession, you occupied it. Then tell us what route shall we take and what kind of behavior," he added as he headed into meetings, according to the Philippine broadsheet.

The Philippines was one of the most outspoken critics of China's manoeuvres in the South China Sea, even filing a complaint with the international tribunal in The Hague, which eventually saw the court throwing out China's claims to the territory.

However, Duterte has made attempts to thaw icy ties with China since taking office in 2016, in hopes of drawing increased trade and investment from Beijing. 

Meanwhile, Chinese Premier Li Keqiang reiterated his support for finalizing a code of conduct by 2021 earlier Wednesday. 

"In recent years, overall peace and stability in the South China Sea has been maintained, and is moving toward greater stability," he said in opening remarks at the summit, adding that a code of conduct would be "conducive to free trade."