Philippine President Rodrigo Duterte has ordered troops to occupy and fortify all Philippine-held islands in the disputed South China Sea to assert its claims amid what he says is a race to control territory.

Duterte made the announcement on Thursday during a televised visit to a military camp on the western island of Palawan, near the disputed Spratly group of islands.

"It looks like everybody is making a grab for the islands there, so we better live on those that are still vacant," he said, adding his country was claiming "nine or 10" Spratly islands, reefs and cays.

"At least, let us get what is ours now and make a strong point there that it is ours."

China asserts sovereignty over almost all of the resource-rich South China Sea - despite rival claims from Southeast Asian neighbours - and has rapidly built reefs into artificial islands capable of hosting military aircraft.

Duterte also said "bunkers or houses and provisions for habitation" were to be built, but it was unclear how his order could be executed.

Some of the tiny reefs and outcrops would need expensive and logistically difficult reclamation work before structures could be built on them.

The defence department later said nine outcrops "are already in our possession" and occupied by marines, including Thitu island where the Philippine military maintains an airstrip.

"The president wants facilities built such as barracks for the men, water [desalination] and sewage disposal systems, power generators, lighthouses, and shelters for fishermen," the department said in a statement.

OPINION: China's South Asian domination

Duterte has previously sought to improve his nation's relations with China by adopting a non-confrontational approach over their competing claims in the strategically vital waters.

An impeachment complaint has been filed against Duterte that cites, among other things, his alleged failure to protest China's territorial expansion in the South China Sea.

An official at the Chinese embassy in Manila seemed surprised when asked by AFP news agency to comment on Duterte's declaration, but referred questions on the matter to the Chinese foreign ministry in Beijing.

The two neighbours are scheduled to hold talks in China in May to tackle issues related to the sea row.

Vietnam, Malaysia, Brunei and Taiwan have also sparred with Beijing over territory in the disputed waterway.

US President Donald Trump's administration has so far taken a tough stance on China's claims in the South China Sea, insisting it will defend international interests there.