Amid ongoing tensions with China, Philippines has warned Beijing that if it attacks its naval vessels in the South China Sea then the South East Asian nation will be forced to invoke its defence agreement with the United States, reports South China Morning Post.

The mutual defence treaty between the US and Philippines was signed in 1951. The treaty mandates that both the countries will support each other in the event of an attack by a third country.

Philippine's foreign secretary Teodoro Locsin Jnr on Wednesday (25 August) said that Manila would continue its air patrolling over the South China Sea despite China's calls to stop what it described as "illegal provocations".

“They [China] can call it illegal provocations, you can’t change their minds. They already lost the arbitral award,” Locsin said, referring to the 2016 decision by an international tribunal that ruled against Beijing’s expansive claims to the South China Sea.

“[But if] something happens that is beyond incursion but is in fact an attack on say a Filipino naval vessel … [that] means then I call up Washington DC,” he added.

The development comes a week after China confiscated a Filipino fishing vessel near the Scarborough Shoal which Beijing has occupied since 2012.

Philippine foreign ministry filed a diplomatic protest with Beijing over the “illegal confiscation” of Filipino fishing equipment.

However, Beijing in response of the Manila's protest said that China’s patrols in the South China Sea were “beyond reproach” and air patrols by the Philippines infringed on its sovereignty.

Chinese foreign ministry spokesman Zhao Lijian defended Chinese coastguard's activities in the region and urged the Philippines to “immediately stop … illegal provocations”.

It should be noted that China claims historic rights over most of the South China Sea, which has been challenged by the Philippines, Malaysia, Vietnam, Taiwan and Brunei.