"The US stands with our Philippine ally in rejecting China's destabilizing plans to establish a 'national nature reserve' at Scarborough Reef," Rubio wrote on X.

"This is yet another coercive attempt to advance China's interests at the expense of its neighbors and regional stability," he wrote.

China on Wednesday announced plans for a reserve to maintain "diversity, stability and sustainability of the natural ecosystem of Huangyan Island," using its term for the contested chain of reefs.

The Philippines said it would issue a protest and charged that China was looking at exerting control rather than protecting the environment.

Scarborough Shoal lies 240 kilometers (150 miles) west of the Philippines' main island of Luzon and nearly 900 kilometres from Hainan, the nearest major Chinese land mass.

China claims almost the entirety of the South China Sea, through which more than 60 percent of global maritime trade passes, despite a 2016 court ruling that said its claims had no basis in international law.

The United States has repeatedly backed the Philippines, its former colony with which it has a defense treaty, despite recent calls by President Donald Trump for strong relations with Beijing.