BEIJING China expressed alarm on Thursday about an agreement in which the Philippines will lease five aircraft from Japan to help patrol the disputed South China Sea.
Philippine President Benigno Aquino said the five TC-90 training aircraft would help the navy in patrolling what the Philippines views as its territory.
The Philippines has made the modernization of its air and naval forces a priority as China deploys missiles and fighters on a number of artificial islands in the South China Sea.
China claims almost the entire South China Sea, where about $5 trillion worth of ship-borne trade passes annually. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims to parts of the sea.
"If the Philippines' actions are to challenge China's sovereignty and security interests, China is resolutely opposed," Chinese Foreign Ministry spokesman Hong Lei told a daily news briefing.
"I also want to point out that Japan is not a party to the South China Sea issue and we are on high guard against its moves. We demand that Japan speak and act cautiously and not do anything to harm regional peace and stability."
China, the world's second-largest economy, and Japan, the third largest, have a difficult political history, with relations strained by the legacy of Japan's World War Two aggression and conflicting claims over a group of uninhabited East China Sea islets.
The Philippine military, for decades preoccupied with domestic insurgencies, has been shifting its focus to territorial defense, allocating 83 billion pesos ($1.77 billion) until 2017 to upgrade and modernize its air force and navy.
Allies the United States and South Korea have already offered help to bolter air capabilities and Aquino announced the arrival this year of two refurbished C130 transport planes from the United States.
Already in the Philippines military's plans is the acquisition of a squadron of multi-role fighters, air-to-ground missile batteries, early warning aircraft and drones.
Last week, the Philippines and Japan signed a deal on the transfer of military equipment and technology, a document Japan needs to allow it to export weapons and guarantee they will not be transferred to a third party.
(Reporting by Ben Blanchard)