The United States and the Philippines will double the size of their annual war games this month, with some exercises to be staged close to a South China Sea flashpoint, the Filipino military said Monday.

The 10-day exercises between the long-time allies will be held as fears grow in the Philippines that China is seeking to take control of the strategically vital and resource-rich sea.

Nearly 12,000 soldiers will be involved in this year's edition in several locations in the Philippines, including a naval station directly facing the disputed waters, military spokesman Lieutenant Colonel Harold Cabunoc said.

That number, which includes 6,600 American troops, compares with a total of 5,500 soldiers who participated in last year's Balikatan (shoulder-to-shoulder) exercises, Cabunoc said.

He emphasised the expanded war games highlighted the deepening military alliance between the Philippines and its former colonial ruler.

"The higher strength of Balikatan 2015 for this year only reflects the Philippines' and the United States' growing commitment to enhance our capability to conduct joint military and non-military activities," Cabunoc told AFP.

He said the decision to expand the numbers involved in the games was not directed at China, which claims nearly all of the South China Sea, even waters and reefs close to Southeast Asian nations and far from its nearest major landmass.

However, part of the exercises will be held from Zambales naval base, which is located 220 kilometres (137 miles) east of Scarborough Shoal in the South China Sea.

The shoal is a rich fishing ground within the Philippines' exclusive economic zone but has been controlled by China since 2012.

The Balikatan exercises, which start on April 20, will also be held on the central island of Panay, Palawan in the southwest and a former American airbase north of Manila.

Cabunoc said the exercises involved maritime security and disaster response drills, as well as civic projects.

Spokespeople for the US and Chinese embassies in Manila were unavailable for comment on Monday.

The Philippines has repeatedly protested at China's increasingly assertive actions in the South China Sea and has sought closer military ties with the United States in an effort to counter it.

The US and Philippines signed an agreement last year that will allow a larger American military presence in the Philippines.

However it has not yet been implemented, as the Supreme Court is hearing challenges to it from anti-US groups.

Aside from the Philippines, Vietnam, Malaysia, Brunei and Taiwan also have competing claims to parts of the South China Sea.