China is rapidly building an airstrip on an artificial island in disputed South China Sea waters, recent satellite pictures show, potentially ramping up tensions with several Southeast Asian neighbours.

Fiery Cross in the Spratly Islands was little more than a reef when China began land reclamation works to turn it into an island in late 2014.

Now satellite images taken last week by DigitalGlobe and shown on the website of the Washington-based Center for Strategic and International Studies (CSIS) show the runway -- estimated at 3.1 kilometres (1.9 miles) in total -- more than one-third complete, it says.

When in operation, it says, it will be able to "accommodate almost any type of aircraft that China would want to land".

"Before this construction China lacked the refuelling and resupply capabilities to reach the southern part of the South China Sea," it added.

"While they have not yet been built, Fiery Cross should be big enough to accommodate hangar facilities for Chinese aircraft."

Pictures taken less than four weeks earlier showed two sections of 468 metres and 200 metres were under construction, CSIS said, demonstrating the speed of the works.

On Wednesday, defence journal IHS Jane's reported that pictures taken by Airbus Defence and Space on March 23 showed a section more than 500 metres long and 50 metres wide.

China claims nearly all of the South China Sea, on the basis of lines on Chinese maps published in the 1940s and locking it into disputes with several Southeast Asian neighbours.

Its island-building in the Spratlys, also claimed in whole or part by the Philippines and Vietnam among others, has been seen as part of an attempt to assert its territorial claims by establishing physical facts in the water.

Fiery Cross is known as Yongshu to Beijing, Kagitinan to Manila, and Da Chu Thap to Hanoi.

Images published this month on the website of the CSIS also showed a flotilla of Chinese vessels dredging sand onto a feature known as Mischief Reef.

That reef is about 100 kilometres (60 miles) from the southwestern Philippine island of Palawan, and roughly 1,000 kilometres from the nearest major Chinese landmass.

- International outcry -

Vietnam, the Philippines and Malaysia have asserted their own claims in the area by stationing troops in the Spratlys and building airstrips there from the 1970s onwards.

Philippine defence ministry spokesman Peter Galvez told AFP Friday the airstrip would have "economic, environmental, diplomatic, defence", and other implications for the Philippines, but did not elaborate.

"As we have said before, all these developments will have a grave impact across multiple dimensions of national security in the immediate to the long term," he said.

Chinese foreign ministry spokesman Hong Lei, in a Friday briefing, maintained that China's island construction was "mainly for the sake of improving the relevant functions of these islands and reefs, and improving the conditions of workers on the islands and reefs".

Such construction was also aimed at "improving search and rescue, environmental protection, security of sea lanes, and safety of fishing activities," he added.

Vietnamese foreign ministry spokesman Le Hai Binh said Thursday that construction by other countries in the Spratlys without permission from Hanoi was "totally illegal and baseless".

Last November, the US warned that the Fiery Cross project could accommodate an airstrip.

"We urge China to stop its land reclamation program, and engage in diplomatic initiatives to encourage all sides to restrain themselves in these sorts of activities," military spokesman Lieutenant Colonel Jeffrey Pool said.

China's moves in the region should spark fear around the world, with military conflict possible, Philippine President Benigno Aquino told AFP on Tuesday.

Beijing quickly dismissed his comments as "groundless".