The Philippines voiced alarm Monday about Chinese "aggressiveness" in disputed regional waters as it launched giant war games with the United States that were partly aimed as a warning shot to Beijing.

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".

He said this was causing concern "not only because it would deter freedom of navigation, but also due to its possibility of military purposes".

China claims sovereignty over most of the resource-rich and strategically important sea, including areas close to other Asian nations, using vague demarcation lines that first appeared on Chinese maps in the 1940s.

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.

In efforts to deter China, the militarily weak Philippines has encouraged longtime ally the US to increase its presence on Filipino soil and coastal waters through expanded and more frequent war games.

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.

- China shadow -

This year's exercises 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.

"I am sure that this Balikatan exercise will... likewise zero in on enhancing our combined capacity to undertake humanitarian assistance and disaster response as well as in dealing with maritime security challenges," Philippine Defence Secretary Voltaire Gazmin said.

US Ambassador to the Philippines Philip Goldberg offered reassuring comments to the Philippines.

"We make no pretence that we are helping the Philippines as it builds a minimal credible defence and protects its maritime security," Goldberg said.

"Let us be clear: the US is committed to its alliance and in the case of the Philippines, our oldest in the region, that commitment is, as President (Barack) Obama has said, is ironclad," Goldberg added.

"At the same time, the US will defend the important principles of freedom of navigation in the air and the sea and the peaceful resolution of disputes through legal, diplomatic and peaceful means."

The term "freedom of navigation" is loaded in the context of the South China Sea, which hosts roughly 40 percent of all the world's shipping trade.

The US has repeatedly expressed concern about the territorial rivalries threatening "freedom of navigation", which angers China as it insists it will always allow ships to sail freely.

China also believes US complaints on the issue are a way for the superpower, which has no territorial claims in the sea, to become involved in the power struggle.

The photographs released by the Philippines further bolster satellite photos from a US-based company released in early April showing a flotilla of Chinese vessels dredging sand onto Mischief Reef -- another outcropping claimed by Manila.

The 10-day war games days will be held at various military bases around the Philippines.

On Tuesday, marines will conduct beach landing exercises from a naval base on the South China Sea, just 220 kilometres (140 miles) from a Philippine-claimed shoal that China has controlled since 2012.

Aquino's spokesman Herminio Coloma alleged Monday that a Chinese vessel recently used a water cannon to drive Filipino fishermen away from the shoal.

This year's exercises will also includes 70 Australian defence force personnel.