The United States has described as "provocative and potentially dangerous" new Chinese restrictions of foreign fishing vessels in disputed waters in the South China Sea.
From January 1, China has required foreign fishing vessels to obtain approval to enter waters it says are under its jurisdiction. It rejects territorial claims by the Philippines, Taiwan, Malaysia, Brunei and Vietnam.
Jen Psaki, a spokeswoman for the US State Department, said on Thursday that China gave no justification under international law for the new restictions.
"Our long-standing position has been that all concerned 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 passing of these restrictions on other countries' fishing activities in disputed portions of the South China Sea is a provocative and potentially dangerous act."
The US is already at odds with China declaring in November an air defence zone over an area of the East China Sea claimed by Japan and South Korea.
The US flew B-52 strategic bombers into the new zone in defiance, raising tensions further in the Pacific.
The new Chinese rules do not outline penalties, but the requirements are similar to a 2004 national law that says boats entering Chinese territory without permission can have their catch and fishing equipment seized and face fines of up to $82,600.
Hua Chunying, a Chinese foreign ministry spokeswoman, said regulating the use of marine resources was a normal practice.
China's ties with the Philippines have been especially frosty over the South China Sea.
Raul Hernandez, a spokesman for the Philippine foreign ministry, said Manila had asked its embassy in Beijing to get more information on the rules.