A coast guard spokeswoman said on Sunday that the Philippines has set navigational buoys within its exclusive economic zone (EEZ) to establish sovereignty over the disputed Spratly islands in the South China Sea.

The move coincides with China’s increasingly assertive behavior in the South China Sea, while Philippines President Ferdinand Marcos Jr seeks closer ties with treaty partner the United States.

The Philippine Coast Guard (PCG) said it set up five buoys carrying the national flag from May 10 to 12 in five areas within the 200-mile (322-km) zone, including the Whitsun Reef, where hundreds of Chinese maritime vessels moored in 2021.

“This move highlights the Philippines’ unwavering resolve to protect its maritime borders and resources and contribute to the safety of maritime trade,” Commodore Jay Tarriela, the coast guard spokesperson on South China Sea issues, said on Twitter.

China’s embassy in Manila did not immediately respond to a request for comment.

In May 2022, the coast guard installed five navigational buoys on four islands in the Spratlys.

China’s claim of sovereignty over almost the entire South China Sea was invalidated by an international arbitration ruling in 2016.

Brunei, Malaysia, Taiwan and Vietnam also have claims in the Spratlys, where China has dredged sand to build islands on reefs, and equipped them with missiles and runways.

Beijing has for years deployed hundreds of coast guard and fishing vessels in disputed areas.