China on Wednesday landed two test flights on an island it has built in the South China Sea, four days after it angered Vietnam with a landing on the same runway in the disputed territory, the Xinhua state news agency said.

The two flights are likely to spark further condemnation from Vietnam, which launched a formal diplomatic protest over the weekend, and the Philippines, which said it was planning to do the same.

Both countries have claims to the area that overlap with that of China, which claims almost the whole of the South China Sea.

Xinhua said the two planes landed on an artificial island in the Spratly Islands on Wednesday morning.

"The successful test flights proved that the airport has the capacity to ensure the safe operation of large civilian aircraft," Xinhua said, adding that the airport would facilitate the transport of supplies, personnel and medical aid.

Xinhua did not give any more detail about what type of aircraft had landed.

The runway at the Fiery Cross Reef is 3,000 meters (10,000 feet) long and is one of three China has been building for more than a year by dredging sand up onto reefs and atolls in the Spratly archipelago.

On Saturday, China landed a civilian plane on the same runway in the Spratlys in its first test, which was also the first time it had used a runway in the area.

The United States has criticized China's construction of the islands and worries that it plans to use them for military purposes, even though China says it has no hostile intent.

The United States said after the first landing it was concerned that the flight had exacerbated tension.

The runways would be long enough to handle long-range bombers and transport aircraft as well as China's best jet fighters, giving it a presence deep in the maritime heart of Southeast Asia that it has lacked until now.

More than $5 trillion of world trade is shipped through the South China Sea every year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.

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