China landed two more planes on a contested reef in the South China Sea Wednesday, state media said, despite international condemnation of a landing at the same location days earlier.

Two civilian aircraft landed Wednesday morning on Fiery Cross reef in the disputed Spratlys island group during "test flights", the official Xinhua news agency said. Vietnam also claims the reef.

The planes departed from and returned to the city of Haikou, the capital of the southern island province of Hainan -- a two-hour journey each way.

"This successful test flight proves that this airport is equipped with the capacity to ensure the safe operation of large civilian aircraft," said Xinhua.

It said the facility would help transport supplies, personnel and medical aid.

China claims virtually all 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.

Several other claimants have also built facilities but at a slower pace, and China's activities have heightened tensions in the region.

It began work in 2014 on the 3,000-metre (9,842 feet) runway on Fiery Cross reef, around 1,000 kilometres (620 miles) from Hainan.

Last Saturday China said it had landed a civilian plane on the runway in an initial test flight -- the first time it had been used.

That landing sparked a formal diplomatic complaint from Hanoi, which labelled it a violation of sovereignty.

The Philippine Department of Foreign Affairs has also said it would file a protest at the weekend incident.