Growing tensions:Chinese vessels anchored at Whitsun Reef in the South China Sea on March 31.AFP  

The Philippines defence department said on Thursday it was keeping its options open as a diplomatic row with Beijing grows over hundreds of Chinese vessels in the contested South China Sea.

Tensions over the resource-rich waters have spiked in recent weeks after over 200 Chinese boats were detected at Whitsun Reef in the Spratly Islands, where China and the Philippines have rival claims. China, which claims almost the entirety of the sea, has refused repeated appeals by the Philippines to withdraw the vessels, which Manila says unlawfully entered its exclusive economic zone.

While President Rodrigo Duterte has appeared reluctant to confront China on the issue, one of his top aides warned on Monday that the boats could ignite “unwanted hostilities”. “As the situation evolves, we keep all our options open in managing the situation, including leveraging our partnerships with other nations such as the United States,” Philippine defence department spokesman Arsenio Andolong said on Thursday.

Mr. Andolong’s comments came after the United States reminded China of Washington’s treaty obligations to the Philippines in the event of an attack in the waters. “An armed attack against the Philippines’ armed forces, public vessels or aircraft in the Pacific, including in the South China Sea, will trigger our obligations under the U.S.-Philippines Mutual Defense Treaty,” State Department spokesman Ned Price told mediapersons on Wednesday.

“We share the concerns of our Philippine allies regarding the continued reported massing of PRC [People’s Republic of China] maritime militia near the Whitsun Reef,” Mr. Price said.