The weeks-long operation, an opening salvo of the broader Balikatan exercises that will next month include the countries' navies and air forces, comes as tensions with China over the disputed South China Sea are at a high.

The two armies will focus "on enhancing combined/joint all-domain operations, reinforcing warfighting capabilities, and showcasing a strong bilateral commitment to safeguarding the Philippines' territorial integrity", the military said in a statement.

Hegseth is scheduled to meet his counterpart Gilberto Teodoro on Friday in Manila, part of a trip that will also take him to treaty ally Japan and the World War II battleground island of Iwo Jima.

It marks his first visit to Asia since his appointment by President Donald Trump and Senate confirmation hearing.

Manila and Washington, longstanding treaty allies, have deepened their defence cooperation since Philippine President Ferdinand Marcos took office in 2022 and began pushing back on Beijing's sweeping claims to the South China Sea.

In 2023, the two countries inked a deal to expand US access to an additional four military bases, bringing the total to nine across the archipelago nation.

In December, the Philippines angered China when it said it planned to acquire the US mid-range Typhon missile system in a push to secure its maritime interests.

The US Army deployed the system in the northern Philippines earlier in 2024 for annual joint exercises, then left it there despite criticism by Beijing.

Hegseth's predecessor Lloyd Austin predicted in November that the Philippines would remain a key ally after Trump's inauguration.

"The strength of our alliance, I think, will transcend changes of administration going forward," Austin said at a press briefing on the western Philippine island of Palawan, which faces the South China Sea.