In a development that is expected to further rile China, Australia and the Philippines announced Saturday the start of joint air and sea patrols in the West Philippine Sea, which is developing into a hotbed of conflict in the region.

The three-day Maritime Cooperative Activity from 25 to 27 November came days after the United States held a similar exercise with the Philippines in the West Philippine Sea, which Beijing almost entirely claims.

China put the People's Liberation Army's naval force on high alert last Thursday when the joint maritime patrol of the Philippines and US navies was launched under an interoperability program.

In a joint statement, the Australian and Philippine defense chiefs said the three-day patrols showed their "shared commitment to exercising freedom of navigation and overflight consistent with international law."

President Ferdinand R. Marcos Jr. said the bilateral maritime activity with Australia will elevate both nations' strategic partnership in the wake of the state visit of Australian Prime Minister Anthony Albanese last September.

"We endeavor to enhance bilateral interoperability in maritime security and domain awareness; test doctrines, existing protocols, and enhance efficiency; and foster closer cooperation between our countries' armed forces," Marcos said.

"This inaugural maritime cooperative activity and those that may follow are a practical manifestation of the growing and deepening strategic defense partnership between our countries," he added.

The joint patrol, Marcos said, highlights the shared commitment of the Philippines and Australia to "support the rules-based international order and a more peaceful, secure, and stable Indo-Pacific region."

The Philippines will deploy two Navy vessels and five surveillance aircraft to join Australia's HMAS Toowoomba warship and one P-8A maritime surveillance aircraft.

The maritime patrols will be held within the Philippines' exclusive economic zone, Department of National Defense spokesperson Arsenio Andolong said.

In September, Albanese and Marcos signed a strategic partnership agreement to expand cooperation in several areas, from defense and security to climate change and education.

The long-anticipated patrols with the United States and Australia follow a spike in tension between Beijing and Manila over their territorial dispute.

China has ignored an international tribunal ruling that said its claims have no legal basis. It deploys boats to patrol the sea and has built artificial islands that it has militarized to reinforce its claims.

On 19 November, Marcos warned that the Chinese military has "started to show interest" in building bases on reefs that are "closer and closer to the Philippine coastline."