Manila (AFP) - 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.

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.

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