A Chinese air defense identification zone over artificial islands that Beijing is building in the South China Sea will be a “serious cause for concern” for the Philippines, the Department of Foreign Affairs (DFA) said on Monday.

China has not yet declared an air defense zone, but a declaration is seen as likely as Beijing has begun challenging foreign military and commercial aircraft flying over the artificial islands it is building in the Spratly archipelago.

On April 20, a Chinese Coast Guard vessel fired an illumination round at a Philippine Navy surveillance plane flying over Philippine-occupied Pag-asa Island in the Spratlys and radioed the pilots, warning them that they were flying through Chinese airspace.

The pilots ignored the Chinese warning and went on with their mission, according to the Armed Forces of the Philippines.

On May 20, the Chinese Navy challenged a US Navy P-8 Poseidon surveillance plane flying over Philippine-claimed Kagitingan Reef (Fiery Cross Reef) with a CNN news crew on board, warning the pilots that they were flying through Chinese airspace and telling them to go away.

The American pilots radioed back that they were flying through international airspace.