China on Saturday vowed to continue building in disputed reefs of the South China Sea, as state media said construction had finished on two lighthouses on reefs claimed by other countries.

Beijing has been bolstering its claim to almost all the South China Sea by rapidly building large artificial structures resembling islands, straining ties with neighbours.

The lighthouses on Cuateron Reef and Johnson South Reef in the Spratly islands have been officially opened, the state-run Xinhua news agency said late Friday.

China established control of the Johnson South Reef after a skirmish with Vietnam in 1988. The Philippines claims both reefs as part of its territory.

The lighthouses are 50 metres high, Xinhua said. State television grave prominence to images of the white-structures standing above blue waters.

Chinese foreign ministry spokeswoman Hua Chunying said in an online statement that they would improve "safety of navigation" for ships.

"Next, China will continue to build other civil facilities on occupied islands reefs in the Nansha Islands," she added using China's name for the Spratlys.

China's building has increased tensions with Washington, which has condemned the structures.

Hua's statement came after reports said that the US would send navy ships close to the islands built by China, testing whether Beijing will defend them.

Beijing claims sovereignty over almost the entire sea, including waters, islands, reefs, shoals and rocky outcrops nearer to other countries.

Brunei, Malaysia, the Philippines and Vietnam -- all members of the Association of Southeast Asian Nations (ASEAN) -- claim parts of the sea, while Taiwan is a sixth claimant.

Chinese President Xi Jinping has said China will not militarise its newly-built structures in the Sea, but satellite images show it has constructed runways capable of use by air force jets.