MANILA (REUTERS) - The Philippines denounced on Thursday (Jan 7) Chinese flights to an artificial island in the disputed South China Sea and said if China was not challenged it was likely to impose an "unacceptable" air defence zone over the area.

China landed two test flights on an island it has built in the South China Sea on Wednesday (Jan 6), four days after its first landing on the 3,000m runway on a reef in the Spratly Islands.

"We are very concerned about the fact that China had already flown their flights to Fiery Cross Reef," Philippine Foreign Secretary Albert del Rosario told a news conference. "If this is not challenged, we will have a situation where China will take a position that an air defence identification zone could be imposed."

China declared such a zone over the East China Sea in 2013, where it has over-lapping claims with Japan. The United States criticised it as dangerous and provocative.

Under the zone's rules, all aircraft are meant to report flight plans to Chinese authorities, maintain radio contact and reply promptly to identification inquiries. US, Japanese and South Korean military aircraft have breached the zone without informing China.

Del Rosario said the Philippines would protest to China about its flights. "These are provocative actions which we need to think about and we need to take positions on," he said.

China claims almost the whole of the South China Sea, through which more than US$5 trillion (S$7 trillion) of world trade passes every year, and has been increasingly assertive in staking its claim.

Brunei, Malaysia, Vietnam, Taiwan and the Philippines have rival claims to parts of the sea, which is believed to have rich deposits of oil and gas.

The runway at the Fiery Cross Reef is one of three China has been building for more than a year by dredging sand up onto reefs and atolls.

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

British Foreign Secretary Philip Hammond, in the Philippines as part of a three-country tour of Asia that included China, said freedom of navigation and overflights were non-negotiable. "They are red line for us," Hammond told the same news conference. "We, as an international maritime and trading nation, enjoy freedom of navigation and overflights in the South China Sea. We expect to continue to exercise those rights," Hammond said.

Britain is a major source of defence equipment for the Philippines.