The US has described new Chinese rules on fishing access to disputed areas of the South China Sea as "provocative and potentially dangerous".
Regulations approved by Hainan province requiring foreign fishing vessels to ask for permission to enter its waters took effect on 1 January.
China claims a U-shaped swathe of the South China Sea, which it puts under the authority of Hainan province.
This swathe overlaps areas also claimed by several South East Asian nations.
The move comes with tensions already high over China's recent establishment of an Air Defence Identification Zone (ADIZ) that covers East China Sea islands claimed and controlled by Japan, and a rock claimed by South Korea.
It also comes as China takes an increasingly assertive stance on its territorial claims across Asia.
"The passing of these restrictions on other countries' fishing activities in disputed portions of the South China Sea is a provocative and potentially dangerous act," state department spokeswoman Jen Psaki said in a regular briefing.
"These regulations appear to apply to the maritime space within China's so-called nine-dash line. China has not offered any explanation or basis under international law for these extensive maritime claims."
The US believed all parties "should avoid any unilateral action that raises tensions and undermines the prospects for a diplomatic or other peaceful resolution of differences", she said.
The rules - which require foreign vessels to obtain permission to fish or survey fisheries resources in waters administered by Hainan from the "relevant" department of China's State Council - were passed by the provincial government in November, China Daily said.
Under existing national law, unauthorised boats that enter Chinese waters can be fined and have their equipment seized.
Hainan province says on its website that its area covers the Paracel and Spratly Islands, which are claimed by Vietnam, the Philippines and Malaysia, and covers about 2m sq/km of the 3.5m sq/km South China Sea.
The Philippines said it was "gravely concerned" by the new rules, which it said threatened regional stability and raised tensions.
"We have requested China to immediately clarify the new fisheries law issued by the Hainan Provincial People's Congress," the Foreign Ministry said in a statement.
Taiwan says it does not recognise the rules. Vietnam, in a written statement to Reuters news agency, said: "All foreign activities at these areas without Vietnam's acceptance are illegal and groundless."
A Chinese foreign ministry spokeswoman, meanwhile, said the rules were "to strengthen the security of fisheries resources", and were "absolutely a normal routine practice".
They come a year after Hainan announced rules that gave police the right to board and seize foreign ships involved in unauthorised entry, damage to coastal defence facilities "and engaging in publicity that threatens national security", according to state-run China Daily.
It is not clear to what extent the new rules would be enforced, given the size of the area involved.
The announcement of China's ADIZ last year sparked concern across the region. It said that aircraft flying through the zone must follow its rules, including filing flight plans.
The zone covers East China Sea islands called Senkaku in Japan and Diaoyu in China that both sides claim. Japan controls the islands, which are also claimed by Taiwan.
The US, Japan and South Korea have rejected China's zone, and flown undeclared military aircraft through it. The US has called the move a unilateral attempt to change the status quo in the region.