Around 17,000 troops are expected to take part in the annual "Balikatan", or "shoulder to shoulder" drills, which for the first time will include an integrated air and missile defence simulation to be attended by President Ferdinand Marcos.

Sophisticated US weapons including the "highly mobile" NMESIS anti-ship missile system will also be deployed, including near a crucial chokepoint in the waters separating the northern Philippines from self-ruled Taiwan.

"We will demonstrate not just our will to uphold our mutual defence treaty in existence since 1951 but our matchless capability to do so," US Marine Corps Lieutenant General James Glynn said Monday at the Balikatan opening ceremony in Manila.

"Nothing builds bonds more quickly than shared adversity," he added, without specifying a common threat.

Philippines Major General Francisco Lorenzo added the exercises would reinforce the country's ability to address "contemporary security challenges".

The Philippines has been engaged in months of confrontations with Beijing over disputed areas of the South China Sea.

It has steadily deepened defence cooperation with treaty ally the United States since Marcos took office in 2022 and began pushing back on China's sweeping claims to the crucial waterway.

During a recent visit to Manila, US Defence Secretary Pete Hegseth said Washington was "doubling down" on the alliance.

"Deterrence is necessary around the world, but specifically in this region, in your country -- considering the threats from the Communist Chinese," he said in late March.

While the bulk of visiting forces involved in Balikatan will be from the United States, countries including Australia and Japan are sending smaller contingents.

- 'More missile systems' -

Last year's Balikatan featured tests of the US Typhon mid-range missile system, which was left behind following the exercises.

The Philippine Army subsequently said it was planning to acquire the Typhon, sparking warnings from China of a regional "arms race".

Chief of Staff Romeo Brawner last month said the Philippines was indeed looking to upgrade its arsenal.

"We are looking at acquiring more missile systems to complete the integration of our air and missile defence," he told New Delhi's Raisina Dialogue, adding more warships and fighter jets were also being sought.

On April 2, the United States said it had approved the possible sale of $5.58 billion in long-coveted F-16 fighter jets to the Philippines, though Manila said the deal was "still in the negotiation phase".

A week later, the Philippines took possession of the first of two corvette-class warships with "advanced weapons and radar systems" acquired in a deal with South Korea's Hyundai Heavy Industries.

- 'Inevitably' involved -

Given the Philippines' proximity to Taiwan and its surrounding waters, Manila's cooperation would be crucial in the event of any US conflict with China.

On April 1, as Chinese ships and warplanes surrounded the self-ruled island in a simulated blockade, Brawner said his country would "inevitably" be involved should the democracy be invaded.

China considers Taiwan part of its sovereign territory and has vowed to one day bring it under its control.

While Manila later said Brawner's comments were primarily referencing efforts to retrieve Filipino workers in Taiwan, its Enhanced Defense Cooperation Agreement with Washington gives US forces access to nine bases in the country.

Two of those are located in Luzon's northernmost Cagayan province, which will host live-fire drills during Balikatan this year.

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