Philippine President Ferdinand Marcos Jr has warned China not to cross a red line in the South China Sea, saying If any Filipino died as a result of Beijing’s wilful actions, Manila would consider it as close to “an act of war” and respond accordingly.

Addressing the defence-and-security focused Shangri La Dialogue in Singapore on Friday night, Mr. Marcos sought to deepen defence cooperation with the U.S. as he stands up to the Chinese military’s aggressive actions in the South China Sea.

“We shall strengthen our alliances with the United States and our strategic partnerships with Australia, Japan, Vietnam, Brunei, and all the other member states of ASEAN. We will also pursue more robust collaborations with countries such as the Republic of Korea, India amongst others,” Mr. Marcos said.

China claims nearly all of the South China Sea, though Taiwan, the Philippines, Brunei, Malaysia and Vietnam claim parts of it.

Mr. Marcos said the Philippines and other Southeast Asian countries had a vision for “peace, stability, and prosperity” in the South China Sea, but that this was being undermined by other actors.

“Unfortunately, this vision remains for now a distant reality. Illegal, coercive, aggressive, and deceptive actions continue to violate our sovereignty, sovereign rights, and jurisdiction,” Mr. Marcos said.

“If a Filipino citizen was killed by a wilful act, that is very close to what we define as an act of war,” he said.

“We would have crossed the Rubicon. Is that a red line? Almost certainly,” he asserted.

China’s determining influence over the security situation and the economic evolution of this region is a permanent fact, he said.

“At the same time, the stabilising presence of the United States is crucial to regional peace. It’s never a choice. Both countries are important,” he underscored.

Mr. Marcos pointed out that security in the South China Sea, through which a huge volume of trade passes, is a global issue.

Mr. Marcos said tensions between the U.S. and China were destabilising for Southeast Asia, calling on Washington and Beijing to work harder to resolve disputes.

“Their rivalry is constraining the strategic choices of regional states. Their contest is exacerbating flashpoints and has created new security dilemmas,” Mr. Marcos said.

“The continued stability of this region requires China and the United States to manage their rivalry in a responsible manner,” he said.

Manila and Beijing have a long history of maritime territorial disputes, but tensions have worsened under Mr. Marcos, who has insisted the Philippines will not give up a “square inch of our territory”.

In recent months the simmering dispute between China and the Philippines over territory in the South China Sea has sharpened into aggressive clashes. Manila has denounced Chinese patrol ships firing water cannons at Philippine boats and supply vessels, injuring some personnel.

Mr. Marcos said on Friday that the South China Sea dispute is an issue that goes beyond the Asia-Pacific region.

The peace and stability of the South China Sea and the freedom of navigation in the South China Sea is a world issue, Mr. Marcos said.

The Philippines, a treaty ally of the U.S., is a key focus of Washington’s efforts to strengthen alliances and partnerships in the Asia-Pacific region as it seeks to counter China’s growing military might and influence.

Given its position in the South China Sea and proximity to self-ruled Taiwan, which China claims as its own, Philippine support would be crucial for the U.S. in the event of any conflict.

The Philippines expanded a 2014 agreement last year to give U.S. military access to another four of its military bases, taking the total number to nine, including two in the far north of the country, less than 450km from Taiwan.

The Enhanced Defence Cooperation Agreement (EDCA) also allows U.S. troops to rotate through and store defence equipment and supplies.

The U.S.’ growing presence in the region and its regular deployment of warships and fighter jets in the Taiwan Strait and South China Sea have infuriated Chinese leaders in Beijing.

China has accused the U.S. of using the Philippines as a “pawn to stir up trouble in the South China Sea”, according to U.S.-China affair watchers and analysts.

China’s Defence Minister Admiral Dong Jun is scheduled to address the annual Dialogue on Sunday and is expected to respond to China-related issues on the South China Sea.

The security forum in Singapore was attended by U.S. Defence Secretary Austin and defence chiefs from around the world.