The United States and the Philippines are expected to announce a deal Thursday that will give US troops access to another four bases in the Southeast Asian nation, as the longtime allies seek to counter China's military rise.

The agreement to expand cooperation will be announced during a visit by US Defense Secretary Lloyd Austin, a senior Philippine official told AFP.

It comes as the countries seek to repair ties that were fractured in recent years -- previous Philippine president Rodrigo Duterte favoured China over his country's former colonial master, but the new administration of Ferdinand Marcos has been keen to reverse that.

China's growing assertiveness on Taiwan and its building of bases in the disputed South China Sea have given fresh impetus to Washington and Manila to strengthen their partnership.

Given its proximity to Taiwan and its surrounding waters, the Philippines' cooperation would be key in the event of a conflict with China, which a four-star US Air Force general has warned could happen as early as 2025.

"There's been an agreement to designate four new additional sites," the Philippine official told AFP on the condition of anonymity.

Talks were ongoing for a potential fifth base, the official added.

The two countries have a decades-old security alliance that includes a mutual defence treaty and a 2014 pact, known as EDCA, which allows US troops to rotate through five Philippine bases, including those near disputed waters.

It also allows for the US military to store defence equipment and supplies on those bases.

EDCA stalled under Duterte, but Marcos has sought to accelerate its implementation.

Under the EDCA expansion to be unveiled Thursday, the United States will have access to at least nine military bases across the archipelago.

It has been widely reported that most of the new bases will be on the main island of Luzon -- the closest Philippine landmass to Taiwan -- where the US already has access to two sites.

The fourth will reportedly be on the western island of Palawan, facing the Spratly Islands in the hotly contested South China Sea, taking the number of sites there to two.

- 'Key' ally -
Ahead of the expected announcement, Austin met Marcos at the presidential palace, where the Pentagon chief described the Philippines as a "key" US ally.

Austin said the United States would continue to help "build and modernise" the Philippine military's capability, and increase interoperability between their forces.

While Marcos has sought to strike a balance between China and the United States, he has insisted he will not let Beijing trample on Manila's maritime rights.

A senior US defence official told reporters Wednesday that the Philippines was under "day-to-day pressure from (China) in ways that contravene international law".

The United States aims to ensure "they have the capability to defend their own sovereignty," the official said.

About 500 US military personnel are currently in the Philippines, with others rotating through the country for joint exercises conducted throughout the year.

Beijing claims sovereignty over almost the entire South China Sea and has ignored a ruling at The Hague that its claims have no legal basis.

The Philippines, Vietnam, Malaysia and Brunei also have overlapping claims to parts of the sea.

China also claims self-ruled, democratic Taiwan as part of its territory, to be reclaimed one day, by force if necessary.

"Looking at the location of the proposed sites, it seems pretty clear that these sites are in relation to a Taiwan contingency," said Greg Wyatt of PSA Philippines Consultancy.