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.

Since then, diplomatic relations between Manila and Beijing have deteriorated as the Marcos administration pushes back against Chinese actions.

China deploys coast guard and other vessels to patrol waters around reefs claimed by the Philippines.

That includes Second Thomas Shoal, which lies about 200 kilometres (120 miles) from the western Philippine island of Palawan and more than 1,000 kilometres from China's nearest major landmass, Hainan island.

The Philippines deliberately grounded a navy ship on the reef in 1999 to assert Manila's claims to the area.

A handful of Filipino troops stationed on the rusty vessel rely on the regular delivery of provisions for their survival.

Dozens of Filipino activists rallied in Manila on Friday to insist that the West Philippine Sea -- the Philippines' name for the South China Sea waters to its immediate west -- "is ours".

- 'Deeply concerned' -

US Secretary of State Antony Blinken said in a statement for the anniversary that Washington remained "deeply concerned" about China's claims "over vast areas that are clearly within the maritime jurisdiction of Vietnam, the Philippines, Malaysia, and Brunei".

Blinken called on Beijing to "abide by the 2016 arbitral ruling" and "cease its dangerous and destabilising conduct".

The Philippines has deepened defence cooperation with the United States and other countries in the face of China's growing assertiveness.

On Monday, the Philippines signed a key defence pact with Japan that will allow the deployment of troops on each other's territory.

Ano said Friday the government would continue to "foster closer ties with like-minded countries" and remained open to discussing "difficult issues".

The Philippines was "always open to... frank discussion based on mutual respect and sincerity," Ano told the gathering that included ambassadors from the United States, Australia, France and Japan.

"If you call, we will answer."