Thousands of American and Filipino soldiers on Monday will kick off expanded wargames, showcasing a deepening defence alliance as alarm bells ring over China.

The 10-day exercises in the Philippines are an annual affair between the longtime allies, but this time they will involve double the number of soldiers as last year in a sign of their expanding military partnership.

The Philippines is seeking more US military and diplomatic support to fend off China's increasing forcefulness in its bid to assert sovereignty in disputed areas of the South China Sea.

In an interview with AFP last week, Philippine President Benigno Aquino said the two militaries operating side-by-side should offer a "deterrent aspect to any entity, be it a country or Islamic radicals".

Aquino insisted the Balikatan (Shoulder-to-Shoulder) wargames starting on Monday were not directed at China, pointing out they were annual exercises, but he discussed at length the Philippines' reliance on the United States.

Foreign Affairs Secretary Albert del Rosario also said last week the Philippines planned to ask Washington for extra help in containing China.

"We are, at this point, seeking additional support from the US in terms of being able to take a stronger position, in defending our position, which is to uphold the rule of law," del Rosario told local broadcaster ANC.

Beijing claims sovereignty over most of the strategically important South China Sea, including areas just off the coasts of 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 sought to expand its presence in disputed parts of the sea in recent years, including by taking control of a shoal that is a rich fishing ground within the Philippines' exclusive economic zone.

It has also embarked on giant reclamation works on reefs and islets, turning some into islands capable of hosting military aircraft landing strips.

- 'Elbowed aside' -

The Philippines has been the most vocal of the rival claimants to express alarm over China's moves.

In his interview with AFP, Aquino warned that the world should fear China's actions in the South China Sea.

US President Barack Obama also expressed concern this month, criticising China for "using its sheer size and muscle to force countries into subordinate positions".

"Just because the Philippines or Vietnam are not as large as China, doesn't mean that they can just be elbowed aside," Obama said.

China has repeatedly insisted it has sovereign rights to the contested areas, so criticism of its reclamation and other activities in the area are baseless.

The US has been looking to re-build its military presence in the Philippines, a former colony where it had naval and air bases until the early 1990s, as part of Obama's "pivot to Asia".

The two nations, bound by a 1951 mutual defence treaty, last year signed another pact that would allow more US forces in the Philippines, although the Supreme Court in Manila still needs to ratify it.

Even without the implementation of the new pact, more than 12,000 soldiers from both sides will be involved in this year's Balikatan exercises, double last year's number.

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

Aside from the naval base, live fire and disaster response drills will also be held in various military camps outside Manila.