The Philippines, the U.S., Australia, and Japan held the first multilateral maritime cooperative activity (MMCA) as territorial tensions mounted in the South China Sea.

The naval drills were conducted on April 7 to 'uphold the right to freedom of navigation and overflight, and respect for maritime rights under international law, reflected in the UN Convention on the Law of the Sea', the four treaty allies said in a joint statement.

The Armed Forces of the Philippines said: 'The MMCA held within the Philippine Exclusive Economic Zone involved naval vessels and aircraft from the four countries. Participants include the BRP Gregorio Del Pilar (PS15) with AW109 helicopter, BRP Antonio Luna (FF151) with AW159 Wildcat ASW helicopter, and BRP Valentin Diaz (PS177) from the Philippine Navy; the USS Mobile and a P-8A Poseidon from the United States Navy; the Royal Australian Navy HMAS Warramunga and Royal Australian Air Force P-8A Poseidon Maritime Patrol Aircraft; and the JS Akebono from the Japan Maritime Self-Defense Forces.

'The MMCA demonstrated the participating countries' commitment to strengthen regional and international cooperation in support of a free and open Indo-Pacific through interoperability exercises in the maritime domain. It will also contribute greatly to the AFP's capability development.'

Communist China's People's Liberation Army also held 'combat patrols' in the South China Sea on the same day.

It said in a statement published on state-controlled media: 'Southern Theater Command of the People's Liberation Army organized a joint maritime and aerial combat patrol on Sunday in the South China Sea.

'All military activities that disturb the stability of the South China Sea are under control.'

Beijing has recently sparked alarm over its increasingly aggressive actions to assert territorial claims in the contested waters.

Expansionist China currently lays claim over almost the entire South China Sea, one of the world's busiest sea lanes. But a United Nations-backed tribunal in 2016 deemed its claims groundless.

The Communist rogue state - accused of genocide for its crimes against the Uyghur population - has been steadily increasing its influence in recent years through investment in developing countries.