Tensions over the strategic waterway, a key passage for sea-borne trade, have soared in the past 18 months following a series of escalating confrontations between Philippine and Chinese ships.

The most serious happened on June 17, when China Coast Guard personnel wielding knives, sticks and an axe surrounded and boarded three Philippine Navy boats during a resupply mission to Second Thomas Shoal in the Spratly Islands.

While the countries agreed last week to "de-escalate tensions" over the disputed reefs and waters, Philippine National Security Adviser Eduardo Ano said Friday that Manila would not back down.

"We will continue to stand our ground and push back against coercion, interference, malign influence and other tactics that seek to jeopardise our security and stability," Ano said, at an event celebrating the anniversary of the ruling by The Hague-based Permanent Court of Arbitration.

The Philippines filed a case against China in 2013. Three years later, the Permanent Court of Arbitration ruled in Manila's favour, declaring China's expansive claims had no legal basis.

China refused to take part in the proceedings and has ignored the judgement.

Former president Rodrigo Duterte, who took office in 2016, set aside the ruling in exchange for warmer ties with China.

That changed when Philippine President Ferdinand Marcos was elected in 2022, insisting he would not let China trample on Manila's maritime rights.