The Philippines Monday began separate but simultaneous naval exercises with the United States and Japan, amid shared and growing concern at Chinese island-building in the disputed South China Sea.

Manila has been holding the naval drills with its longtime ally Washington since 1995. But the exercise with Tokyo, a World War II foe, is only its second ever after one earlier this year.

This week's Cooperation Afloat Readiness and Training (CARAT) drill with Washington will include a P-3 Orion aircraft, of the type used by the US to monitor the South China Sea.

China claims almost the entire Sea despite competing claims from the Philippines, Brunei, Malaysia, Taiwan and Vietnam, and has been taking strong action including reclamation to assert sovereignty.

"CARAT remains a practical way to address shared maritime security priorities, enhance our capabilities, and improve inter-operability between our forces," the US exercise commander, Rear Admiral William Merz, said at the opening ceremony in Puerto Princesa city on the southwestern Philippine island of Palawan.

Rear Admiral Leopoldo Alano, commander of the Philippine Fleet, described the drill as a great opportunity "to gain valuable experience and increase our inter-operability."

The drills will also feature for the first time the littoral combat ship USS Fort Worth, and involve the rescue and salvage ship USNS Safeguard.

While it does not take sides in the dispute, the US has in recent weeks intensified its criticism of China's reclamation work, which has created new islands including airstrips on reefs and shoals also claimed by its neighbours.

The US says the activities could pose a threat to freedom of navigation.

China said last week its land reclamation in the disputed Spratly islands would finish soon but be followed by "facility construction".

The Philippines has asked a United Nations tribunal to reject China's claims to most of the Sea, a move angrily rejected by Beijing which says the world body has no authority in the matter.

This week's naval exercise will be held both on Palawan, the closest land mass to the disputed reefs and waters, and in the Sulu Sea to the east of the island.

The Filipino forces in the drills, including the US-acquired frigates BRP Ramon Alcaraz and BRP Gregorio del Pilar, also regularly patrol the South China Sea.

The exercise will focus on combined maritime operations, mobile dive and salvage training, coastal riverine operations and maritime patrol and reconnaissance along with seminars ashore, the US Navy said.

Japan, which has its own maritime dispute with China in the East China Sea, has also expressed concern at Beijing's reclamation further south.

On Monday it began three days of drills with the Philippine Navy involving a Japanese P-3C patrol aircraft.

The drills, which will also include a Philippine Navy aircraft, will focus on joint search and rescue operations on the high seas, the Philippine Navy said.

They will take place in international airspace and outside Philippine territorial waters, it said in a statement.