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 of China 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.'Groundless'
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.
China establishes 'air-defence zone' over East China Sea
China has outlined an "air-defence identification zone" over an area of the East China Sea, covering islands that are also claimed by Japan.
China's defence ministry said aircraft entering the zone must obey its rules or face "emergency defensive measures".
The islands, known as Senkaku in Japan and Diaoyu in China, are a source of rising tension between the countries.
Japan lodged a strong protest over what it said was an "escalation".
"Setting up such airspace unilaterally escalates the situations surrounding Senkaku islands and has danger of leading to an unexpected situation," Japan's foreign ministry said in a statement.
Taiwan, which also claims the Diaoyu/Senkaku islands, expressed regret at the move and promised that the military would take measure to protect national security.
'No specific target'
In its statement, the Chinese defence ministry said aircraft must report a flight plan, "maintain two-way radio communications", and "respond in a timely and accurate manner" to identification inquiries.
"China's armed forces will adopt defensive emergency measures to respond to aircraft that do not co-operate in the identification or refuse to follow the instructions," said the statement.
It said the zone came into effect from 10:00 local time (02:00GMT) on Saturday.
State news agency Xinhua showed a map on its website covering a wide area of the East China Sea, including regions very close to South Korea and Japan.
Responding to questions about the zone on an official state website, a defence ministry spokesman, Yang Yujun, said China set up the area "with the aim of safeguarding state sovereignty, territorial land and air security, and maintaining flight order".
"It is not directed against any specific country or target," he said, adding that China "has always respected the freedom of over-flight in accordance with international law".
"Normal flights by international airliners in the East China Sea air-defence identification zone will not be affected in any way."
In September this year, Japan said it would shoot down unmanned aircraft in Japanese airspace after an unmanned Chinese drone flew close to the disputed islands.
China said that any attempt by Japan to shoot down Chinese aircraft would constitute "an act of war".
Last month Japan's defence minister, Itsunori Onodera, said China's behaviour over the disputed East China Sea islands was jeopardising peace.
BBC World Service East Asia editor Charles Scanlon says the confrontation over the small chain of uninhabited islands is made more intractable by conflicting claims for potentially rich energy resources on the sea bed.