China's recent landing of aircraft on a contested reef in the South China Sea is raising tensions and promoting instability in the region, the Pentagon warned Thursday.

A Department of Defense spokesman said three civilian flights are now believed to have landed on one of the islands, corroborating Chinese state media reports that three civilian aircraft have landed on Fiery Cross reef in the disputed Spratlys island group.

"We clearly are concerned by these flights... and we're concerned by all of these activities being conducted by the Chinese in disputed islands in the South China Sea," Pentagon spokesman Peter Cook told reporters.

"Anything being done by any country to try and raise tensions over these disputed islands, and to try and militarize or engage in reclamation activities in these islands, we think only adds to instability in the South China Sea."

China claims virtually all of the South China Sea, while the Philippines, Vietnam, Malaysia, Brunei and Taiwan also have partial claims.

China has asserted its claim by rapidly building artificial islands, including airstrips said to be capable of hosting military jets.

"We call for a diplomatic resolution to these issues in the South China Sea and certainly these flights do nothing to foster further stability and understanding in that part of the world," Cook said.

China's initial aircraft landing on Saturday prompted a formal diplomatic complaint from Hanoi, which labelled it a violation of sovereignty.

The Philippines also said it would file a protest.