US State Department spokesman John Kirby reiterated a call for a halt to land reclamation and militarization of outposts in the South China Sea, where China and five other Asian countries have competing territorial claims.

"To begin flight operations at this new airfield in a disputed area raises tensions and threatens regional stability," said Kirby on Monday. He said claimants should instead focus on reaching agreement on acceptable behavior in disputed areas.

"We have made this case clear repeatedly, and we will continue to make it," he added.

China's landing of an aircraft at Fiery Cross Reef has already drawn criticism from its neighbors, among them Vietnam, which said China's move violated Hanoi's sovereignty. The Philippines, a US ally, said Monday it was also considering a formal protest.

Hua Chunying, a spokeswoman for the Chinese Foreign Ministry, said China had deployed a "civil aircraft" on the island to determine whether the new airfield - in what she said was Chinese territory - met with civil aviation standards.

Beijing rejected the protest from Vietnam, saying the operation took place within Chinese territory.

Disputed territory

Tensions have risen in the last two years after China transformed disputed reefs in the Spratly Island chain into islands that rival claimants fear Beijing could use to project its military might far from the Chinese mainland.

The diplomatic tussle marks a continuation of tense exchanges, mainly among China, Vietnam and the Philippines, over the long-disputed and potentially oil- or gas-rich offshore territories also claimed by Taiwan, Malaysia and Brunei.

The South China Sea has also become a growing source of tension between Beijing and Washington, which says it has a national interest in the peaceful settlement of disputes in the area and in freedom of navigation in waters critical for world trade.

In October, the US deployed a guided missile destroyer near a different reef to challenge Beijing's claim of sovereignty in the waters around the artificial islands. China criticized the US action, warning Washington not to repeat the maneuver.

bik/cmk (AP, AFP)