Thousands of Philippine and US marines on Monday began military exercises close to flashpoints in the South China Sea, where Beijing is involved in bitter territorial disputes with its neighbours.

The 12-day amphibious landing exercises -- involving about 3,500 US marines and sailors and 1,200 Filipino counterparts -- were officially launched from the Philippines' western island of Palawan facing the South China Sea.

China claims the sea almost in its entirety. Its increasingly assertive efforts to stake its claims have heightened tensions with neighbours including the Philippines, which has conflicting claims to parts of the waters.

US and Philippine military officials said the launching ceremony and the exercises were not related to the dispute with China.

"Our primary reason is to establish interoperability and to build capacity in case a (natural) disaster occurs," US Marine Brigadier-General Paul Kennedy answered, when asked if the exercises were tied to a potential conflict.

Regional Philippine military chief Admiral Alexander Soria said the two sides were "trying to develop a bilateral force that can respond immediately to disaster situations and that we speak the same language once we do".

Philippine marine spokesman Lieutenant Jerber Anthony Belonio stressed in Manila that the location of the opening ceremony was not linked to the territorial dispute.

"This has no relation whatsoever. This is just to show the capabilities of our new marine landing brigade which coincidentally is based in Palawan," he said.

The Philippines has recently been strengthening its military assets in Palawan.

The poorly-equipped military has also been strengthening its ties with defence allies -- particularly the United States, the former colonial power -- in the face of Chinese sabre-rattling.

The US Marine Corps said the exercises would "enhance the interoperability between US Navy and Marine Corps forces and their Philippine counterparts with a focus on improving our bilateral response to regional issues and maritime security crises".

This year's exercises will include small arms and artillery live-fire training, a mechanised assault, paratroop operations and a simulated boat raid.

- US assault ship arrives -

The amphibious assault ship USS Peleliu, along with two support ships, arrived at the former US Subic naval base at the weekend to take part in the exercises.

In April the allies signed a defence accord giving US forces greater access to Filipino bases as part of a US rebalancing of military power towards Asia.

Although the United States has not taken sides in the territorial disputes, it has warned China against "destabilising actions" in the South China Sea.

The sea is claimed in part by Vietnam, Taiwan, Brunei and Malaysia as well as China and the Philippines.

China has been involved in a string of tense maritime incidents with rival claimants in the sea. Earlier this year it placed an oil rig in waters also claimed by Vietnam, sparking deadly riots in the Southeast Asian nation.