New Chinese coast guard rules took effect Saturday, under which it can detain foreigners for trespassing in the disputed South China Sea, where neighbours and the G7 have accused Beijing of intimidation and coercion.

Beijing claims almost the entirety of the South China Sea, brushing aside competing claims from several Southeast Asian nations including the Philippines and an international ruling that its stance has no legal basis.

China deploys coast guard and other boats to patrol the waters and has turned several reefs into militarised artificial islands. Chinese and Philippine vessels have had a series of confrontations in disputed areas.

From Saturday, China's coast guard can detain foreigners "suspected of violating management of border entry and exit", according to the new regulations published online.

Detention is allowed up to 60 days in "complicated cases", they say.

"Foreign ships that have illegally entered China's territorial waters and the adjacent waters may be detained."

Manila has accused the Chinese coast guard of "barbaric and inhumane behaviour" against Philippine vessels, and President Ferdinand Marcos last month called the new rules a "very worrisome" escalation.

China Coast Guard vessels have used water cannon against Philippine boats multiple times in the contested waters.

There have also been collisions that injured Filipino troops.

Philippine military chief General Romeo Brawner told reporters on Friday that authorities in Manila were "discussing a number of steps to be undertaken in order for us to protect our fishermen".

Philippine fishermen were told "not to be afraid, but just to go ahead with their normal activities to fish there in our Exclusive Economic Zone", Brawner said.

The United States said the "purported regulations" raised significant legal concerns.

Chinese "domestic law has no applicability to other states' flagged vessels in other states' exclusive economic zones or in the high seas," a US State Department spokesperson told AFP. "Enforcement would be highly escalatory and detrimental to regional peace and security."

The official said Washington "unequivocally rejects" China's "sweeping and unlawful maritime claims" in the South China Sea.

"We've urged Beijing –- and all claimants -– to comport their maritime claims with international law," the spokesperson said.