US aircraft dropped bombs and marines tore forward under artillery fire in war games in the Philippines on Thursday, weeks after the allies signed a defence deal against a backdrop of flaring Chinese tensions with its neighbours.

The live rounds made a dull thud and kicked up dust as they rained down on a dry riverbed in the northern Philippines at the start of the hour-long manoeuvres, involving about 100 American and 200 Filipino marines.

"We're training to take over a key enemy position," US Marines spokesman Lieutenant-Colonel Jay de la Rosa told AFP from a nearby ridge observation point, as F/A-18 and A-10 aircraft unloaded their payloads.

Artillery shells also poured down from nearby hilltops before V-22 Osprey aircraft and conventional military helicopters made paratroop drops of marines on the simulated battlefield, later joined by colleagues aboard armoured vehicles.

"It's a maritime security scenario," Filipino Navy Captain Annaleah Cazcarro said.

"We don't have a target country," she emphasised.

Thursday's manoeuvres came at the end of 10 days of annual war games between the US and its close ally the Philippines, involving 5,500 troops and this year addressing security issues in the flashpoint South China Sea.

China is engaged in increasingly tense rows with both the Philippines and Vietnam over the sea, which is believed to harbour vast oil and gas resources and which China claims almost in its entirety.

Thursday's event was held at Crow Valley, a former gunnery range for American forces that were stationed at two nearby large military installations until 1992.

The allies signed a deal last month to give US forces greater access to Filipino bases in the former US colony.

The United States has said it does not take a position on the territorial disputes, but has criticised what it said were "provocative" acts by China to assert its claims.

US President Barack Obama, in a state visit to Manila in late April, also made an "ironclad" pledge to defend the Philippines, with which it has a mutual defence treaty, if attacked.

The Philippines released photographs on Thursday to back its claim that China is reclaiming land on a disputed reef in the South China Sea, in an apparent effort to build an airstrip.

In Vietnam, anti-China riots on Thursday triggered by the communist neighbours' own territorial dispute left a Chinese worker dead and 100 injured.

The Chinese claims to the South China Sea also overlap those of Taiwan, Malaysia and Brunei.