US surveillance aircraft and naval ships have yet to test China's territorial claims around artificial islands built in the South China Sea, but the Pentagon warned Thursday that could be "the next step".

Although the United States does not recognise China's claims of sovereignty around the man-made structures, American P-8 surveillance planes and naval vessels patrolling the area have not ventured within 12 nautical miles of the artificial islands -- the standard territorial zone around natural land.

"That would be the next step," Pentagon spokesman Colonel Steven Warren told reporters.

Asked if the military would move to within that sensitive zone, he said: "We don't have any announcement to make on next steps. We are going to continue our routine flights."

US officials have said they are weighing sending warships and surveillance aircraft within 12 nautical miles of the man-made islands in the South China Sea to test Beijing's controversial territorial claims.

But the move could raise tensions and lead to a standoff on the high seas -- in an area vital to global shipping lanes.

Beijing regards almost the whole of the South China Sea as its own.

The US Navy has released video from a P-8 Poseidon surveillance flight in the South China Sea which received several warnings from the Chinese military.

It showed a flotilla of vessels carrying out reclamation works in one lagoon, and an airstrip under construction on another island.

"You can see here the landing strip and on the back side there is the taxi way," an officer says, pointing at a screen, adding that "hundreds of metres" have been built in "the past couple of months."

The officer explained the huge dredging operation, taking material from the seabed as part of the reclamation project to provide fresh space for construction.

- 'Great wall of sand' -

The new video came after a CNN television crew aboard a P-8 Poseidon plane captured a tense radio exchange between the US aircraft and Chinese forces in the area.

"This is the Chinese navy... This is the Chinese navy... Please go away... to avoid misunderstanding," a voice can be heard telling the Americans.

The Chinese navy issued eight such warnings during the P-8's flight near the Fiery Cross Reef, one of the sites of Beijing's massive land reclamation effort, CNN reported.

American pilots replied in each case that they were flying through "international airspace".

Journalists are rarely allowed to fly in a sophisticated P-8 spy plane, much less permitted to film inside the cockpit, as the CNN crew was.

The Chinese warnings to the US aircraft are typical and occur frequently, a navy official told AFP.

"It's not uncommon," the official said, adding that the Chinese sometimes send military aircraft to visually identify American planes in the area.

China's Global Times newspaper said in an editorial that the access given to CNN showed the US was "trying to sensationalise China's reclamation activities on some reefs and islets in the South China Sea in a bid to impose more pressure on China".

"Washington is purposefully raising tensions with China, a move that has created a higher risk of a physical confrontation between both sides," it added.

Earlier, Beijing said it had "indisputable sovereignty over the Nansha islands and adjacent waters", using its name for the Spratly islands.

"We hope that relevant countries can respect China's sovereignty in the South China Sea, avoid taking actions that may escalate or complicate the matters, and contribute to regional peace and stability," Beijing's foreign ministry spokesman Hong Lei told reporters.

With Beijing pursuing land reclamation at an unprecedented pace, a US naval commander has accused China of building a "great wall of sand" in the South China Sea to bolster its territorial claims.

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