The US has criticised China's creation of a so-called nature reserve in the South China Sea, describing it as an effort to strengthen Beijing's claims over disputed waters.
The US State Department accused China of attempting to deny Filipino vessels access to traditional fishing grounds in the contested area.
Beijing has presented the designation as an environmental measure intended to protect marine ecosystems, and has rejected criticism of its activities in the region.
The Philippines has said the move infringes on its sovereign rights, and Filipino fishing communities have long complained of being driven away from waters they have used for generations.
“Calling it a reserve does not change the reality on the water. The effect is to keep other countries' boats out.”
China claims most of the South China Sea, a position that overlaps with the claims of several neighbouring countries and was rejected by an international tribunal ruling in 2016.
Beijing does not recognise that ruling, and tensions have periodically flared as coastguard and maritime militia vessels confront boats from other claimant states.
The US, a treaty ally of the Philippines, has repeatedly called for freedom of navigation and for disputes to be resolved peacefully under international law.
Regional analysts say environmental designations can serve as a tool for asserting administrative control without the use of overt force.
Fishing is a vital source of income and food for coastal communities in the Philippines, and access to waters around disputed reefs and shoals has become a recurring flashpoint. Filipino officials have documented a series of confrontations at sea in recent years. Beijing, for its part, has accused Manila of provoking tensions and of entering waters it considers Chinese.
Diplomats expect the issue to feature in upcoming regional meetings.
