MANILA - Thousands of American and Philippine troops launched large annual war games yesterday after United States President Barack Obama vowed "ironclad" backing for its Asian ally, which is locked in a tense maritime row with China.

Philippine Foreign Secretary Albert del Rosario said the 10-day drills were necessary to deal with the challenge of "aggressive" neighbours intent on "changing the status quo".

He did not mention China directly, but Beijing has been robust in its efforts to assert territorial claims over most of the South China Sea, putting a strain on its relationships with neighbouring countries.

"In recent years, tensions in the Asia-Pacific region have increased due to extensive and expansive maritime and territorial claims undermining the rule of law," Mr del Rosario said at the opening ceremony. "Aggressive patterns of behaviour aimed at changing the status quo threaten peace and stability in the region."

He added that the military exercise - known as Balikatan (Shoulder To Shoulder) - with its focus on "maritime capability", boosted the Philippines' ability to "address these challenges".

About 2,500 US soldiers are joining 3,000 Filipino troops in the two-week drills on different parts of the main island of Luzon.

The war games will see US F-18 fighters rehearse bombing runs and troops involved in live- fire drills, search-and-rescue operations and humanitarian response scenarios.

There will also be demonstrations of maritime surveillance systems and ship-to-shore landing exercises on the hosts' west coast, facing the disputed waters.

The manoeuvres began a week after Mr Obama assured Manila his government was committed to a 1951 mutual defence treaty.

Dozens of Australian troops, including special forces, were invited this year as the exercise begins to expand to eventually include other key allies in the region.

Looking to raise its regional defence platform, Australia will be bringing P-3 Orion surveillance aircraft to the drills to help boost Manila's "maritime domain awareness", said Wing Commander Nicholas Pratt of the Royal Australian Air Force.

He said the aircraft would be operating out of the western Philippine island of Palawan, fronting the disputed waters.

Also last week, the US and the Philippines bolstered their security alliance with a new agreement giving American forces greater access to Philippine bases - part of a US rebalancing towards rising Asia.

"The (new agreement) updates and strengthens US-Philippine defence cooperation to meet 21st-century challenges," US Ambassador to the Philippines Philip Goldberg said yesterday.

Mr Obama's four-nation Asian tour was dominated by worsening maritime tensions between Beijing and Washington's allies in the region, which have triggered fears of military conflict.

The Philippines, which has one of the weakest militaries in the region, has repeatedly called on the US for help as China has increased military and diplomatic pressure to take control of the contested areas.

While Mr Obama sought to reassure the Philippines that the US would support its ally in the event of an attack, he did not specifically mention coming to the aid of Manila if there were a conflict over the contested South China Sea areas, as his hosts had hoped.

But he ended his trip with a warning to China against using force in territorial disputes.

Mr Obama's trip drew a frosty response from Beijing, with the Chinese authorities accusing the US President of ganging up with "troublemaking" allies.

AGENCE FRANCE-PRESSE, REUTERS