MANILA – The Philippines may tap diplomatic channels to call China's attention to warnings its detachment sent a Philippine aircraft carrying top defense officials approaching Pag-asa Island on Friday.

National Security Adviser Hermogenes Esperon Jr. made the statement after a Chinese station on Subi Reef in the disputed Spratlys challenged four times an aircraft carrying Defense Secretary Delfin Lorenzana, several military officials, and journalists as it approached Philippine-occupied Pag-asa Island.

The official, who also traveled to Pag-asa Island during his time as military chief, said the "challenge" was a normal occurrence, but that the Philippines may "respond appropriately."

“That normally happens when you get to an area where there are overlaps of territory. There will be challenges,” Esperon told reporters after Duterte’s visit to a Russian ship docked in Manila.

Still, he said the warning was “something that’s not very positive.”

“A challenge could be just to identify yourself. But it could also mean you’re challenging because you think that’s your territory,” he said.

“We mind and we will respond appropriately. We have our challenges and answers or protocols on that. It is bad enough and that could be the basis for some note verbale,” he said.

Lorenzana meanwhile said it was standard for the Chinese to do it, and that the Philippine plane responded appropriately to the warning.

The incident followed a report that the Chinese Coast Guard fired shots at a Filipino fishing vessel near the Union Banks, located within the Philippines' exclusive economic zone in the disputed South China Sea.

In July last year, the Philippines won a landmark ruling before the United Nations arbitral tribunal, when it invalidated China's nine-dash line claim covering nearly all of the South China Sea.

China has ignored the ruling and instead continued to fortify its artificial islands in the Spratlys archipelago, asserting "indisputable sovereignty" over the waters.

Since assuming the presidency, Duterte has downplayed Manila’s maritime dispute with China and has chosen to set aside the tribunal’s ruling.

Duterte’s move has improved relations between the two neighbors, even as the tough-talking Filipino leader sought to distance Manila from its long-time ally, Washington.

The Philippines and China are set to hold long-stalled bilateral talks on the dispute in May. – with report from Chiara Zambrano, ABS-CBN News