Longtime allies the Philippines and the United States start annual, large-scale military exercises on Monday after President Barack Obama vowed "ironclad" backing for Manila as a territorial dispute with China simmers.

The two-week Balikatan or "shoulder-to-shoulder" drills involving 5,500 American and Filipino soldiers begin just days after Obama assured Manila his government was committed to a 1951 mutual defence treaty.

The allies last week bolstered their ties with a new defence agreement signed ahead of a visit by Obama giving American forces greater access to Philippine bases -- part of a US rebalancing of military power towards rising Asia.

"What President Obama said was a reaffirmation of our treaty," Philippine military spokesman Lieutenant Colonel Ramon Zagala said.

"It is very important to note that this is really relevant right now that we have a present threat," he added, referring to an increasingly tense row with China over reefs and outcrops in the South China Sea, which Manila calls the West Philippine Sea.

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.

China claims nearly all of the South China Sea, which is believed to contain huge deposits of oil and gas, even waters and islands or reefs close to its neighbours.

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

While Obama sought to reassure the Philippines that the United States 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.

"We believe that nations and peoples have the right to live in security and peace, to have their sovereignty and territorial integrity respected," Obama said.

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

Zagala said troops would be engaged in live-fire drills, search-and-rescue operations and humanitarian response scenarios in several locations.

There will also be demonstrations of maritime surveillance systems and ship-to-shore landing exercises in Zambales province, three hours' drive north of Manila and facing the South China Sea, Zagala said.

"Balikatan 2014 is structured to further develop the AFP (Philippine military) in crisis-action planning, enhancing its ability to effectively conduct counter-terrorism operations and promote inter-operability with the US armed forces," the Philippine military said.

Balikatan is to be officially declared open at 8:30 am (0030 GMT) Monday by officials from both sides, including US envoy to Manila Philip Goldberg and Lieutenant General Terry Robling of the US Marines.