US Defense Secretary Lloyd Austin signed the deal with his Philippine counterpart Gilberto Teodoro at the start of a visit to Manila that will also include a meeting with President Ferdinand Marcos.

The General Security of Military Information Agreement allows for the sharing of classified information that could benefit a US ally's national defence, and streamlines the sale of certain classified technologies, officials said.

Austin and Teodoro did not make any remarks at the signing ceremony, but the Philippine defence department said the document was "a critical step to enhance information sharing and deepen interoperability between the Philippines and the US".

It comes as the Marcos government pushes back against Beijing's territorial claims over most of the South China Sea and as president-elect Donald Trump prepares to return to office.

China has brushed aside an international ruling that its claims have no legal basis, and has deployed navy and coast guard vessels that Manila says harass its vessels and stop them accessing some reefs and islands in the waters.

This has led to violent confrontations that have resulted in injuries to Filipino personnel and damage to their vessels in the past 18 months.

That has sparked concern the United States could be drawn into an armed conflict due to its mutual defence treaty with the Philippines.

The Philippine defence department said Austin is due to visit the western island of Palawan on Tuesday for a meeting with Filipino forces responsible for patrolling the South China Sea and defending outposts.