MANILA » The Philippines has protested China’s efforts to reclaim land in a disputed reef in the South China Sea that can be used to build an airstrip or an offshore military base in the increasingly volatile region, the country’s top diplomat and other officials said Wednesday.

Foreign Secretary Albert del Rosario told The Associated Press that the Philippines lodged the protest against China last month after surveillance aircraft confirmed, and took pictures of the reclamation and dredging being done by Chinese vessels at the Johnson Reef in the Spratly Islands, which Manila says violates a regional non-aggression pact.

China replied to the Philippine protest by saying that the reef belonged to it, he said.

Del Rosario said it’s not clear what China would build on the reef, which Manila claims as part of its western province of Palawan, but one possibility is an airstrip. Another official says China could also build an off-shore military base.

"We’re not exactly sure what are their intentions there," Del Rosario said.

Another senior government official, who spoke on condition of anonymity because he was not authorized to talk about the issue, said China’s reclamation was first detected by air force planes six months ago. Philippine aircraft searching for a missing Malaysian jetliner in March also spotted the continuing reclamation on the submerged Johnson Reef by at least one Chinese ship backed by smaller vessels.

The government estimates that the reclamation has turned the submerged reef and a sand bar into a 30-hectare (74-acre) land mass that transformed the underwater outcrop into an islet, a senior diplomat told The AP on condition of anonymity because of a lack of authority to discuss the issue.

It’s the latest territorial spat between the Asian neighbors that have ratcheted tensions in the potentially oil- and gas-rich region, which also straddles one of the world’s busiest sea lanes. Vietnam and China have separately been engaged in a dangerous standoff off the Paracels Island after Beijing deployed a mobile oil rig backed by dozens of security vessels.

Del Rosario said the Philippines raised the reclamation issue along with the deployment of Chinese coast guard ships at the Second Thomas Shoal and "harassments of our fishermen" during a summit of the Association of Southeast Asian Nations last week in Myanmar. Four members of the 10-nation bloc — Brunei, Malaysia, the Philippines and Vietnam — are locked in territorial disputes in the Spratlys with China and Taiwan.

ASEAN issued a statement expressing concern over recent territorial spats in the South China Sea after the summit attended by Southeast Asian heads of state.

During the summit, Philippine officials also reported the intrusion by a suspected Chinese research ship last month near a Philippine oilfield off Palawan province, del Rosario said.

China and ASEAN member states signed a non-binding 2002 declaration urging rival claimant countries to settle their disputes peacefully, refrain from occupying new islands or reefs and launching construction efforts that could raise tensions.

But accusations of repeated violations of the pact have sparked international calls for a legally binding "code of conduct" that could prevent a major armed conflict in the South China Sea.