The Philippines and Australia began their first joint sea and air patrols in the South China Sea on Saturday, days after Manila took similar steps with the U.S. as Pacific nations warily eye an increasingly assertive China.

Marcos hosts Australian PM Albanese in Manila.

The three-day exercises, announced by Philippine President Ferdinand Marcos Jr on social media, follow discussions by the Philippines and Australia early this year on joint patrols to underscore what they say is their commitment to a rules-based order.

China claims almost the entire South China Sea, a conduit for more than $3 trillion of annual ship-borne commerce, including parts claimed by the Philippines, Vietnam, Indonesia, Malaysia and Brunei. The Permanent Court of Arbitration in 2016 said China's claims had no legal basis.

The Philippines is ramping up efforts to counter what it describes as China's "aggressive activities" in the South China Sea, which has also become a flashpoint for Chinese and U.S. tensions around naval operations.

"Australia and the Philippines are firmly committed to peaceful, secure and prosperous region, where sovereignty and agreed rules and norms are respected," Australian Deputy Prime Minister Richard Marles said in a joint statement posted by Marcos.

"The first joint patrol between the Australian Defence Force and the Armed Forces of the Philippines demonstrates this commitment," Marles said.

The patrols will be carried out in the West Philippine Sea, said Philippine Department of National Defense spokesperson Arsenio Andolong, using Manila's term for waters in the South China Sea that fall within its exclusive economic zone.

The Philippine military said two of its navy vessels and five surveillance aircraft would participate, while Australia would send the frigate Toowoomba and P8-A maritime surveillance aircraft.

"This inaugural Maritime Cooperative Activity and those that may follow are a practical manifestation of the growing and deepening strategic and defense partnership between our countries," Marcos said on X, the platform formerly called Twitter.

The Philippines and the United States concluded three-day joint sea and air patrols on Thursday, starting in waters near Taiwan, a democratically governed island that China claims as its own, and ending in the West Philippine Sea.

China has accused the Philippines of enlisting "foreign forces" to patrol the South China Sea and stirring up trouble. Manila insists the maritime activities are within its rights.