Philippine military chief General Gregorio Catapang released what he said were satellite photos of intense recent Chinese construction over seven reefs and shoals in the Spratly archipelago of the flashpoint South China Sea.
"We have compelling reasons to raise our voice to tell the whole world the adverse effects of China's aggressiveness," Catapang told reporters, describing the reclamation and construction activities as "massive".
Catapang said this was causing concern "not only because it would deter freedom of navigation, but also due to its possibility of military purposes".
The Philippines, Vietnam, Malaysia, Brunei and Taiwan have overlapping claims.
China has expanded its presence in disputed parts of the sea in recent years by embarking on giant reclamation work on reefs and islets, turning some into islands capable of hosting military aircraft landing strips.
In an interview with AFP last week, Philippine President Benigno Aquino said the world should fear China's actions in the disputed sea, warning they could lead to military conflict.
This dovetails with US plans to re-build its military presence in the Philippines, a former colony where it had naval and air bases until the early 1990s.
The Balikatan (Shoulder-to-Shoulder) war games are the biggest annual exercises between the allies, which signed a defence treaty in 1951 committing each to come to the others' aid in the event of external aggression.
This year's event involves about 12,000 troops, double last year's number, and officials from both sides made unsubtle references to the South China Sea in opening ceremony speeches at military headquarters in Manila.
