The Philippines has protested against China's reclamation of land at a disputed reef in the South China Sea that can be used to build an airstrip.
Foreign Secretary Albert del Rosario said Manila filed a diplomatic protest against the reclamation on the Johnson South Reef last month, but Beijing rejected it on grounds that the reef is part of Chinese territory.
Asked if China was building an airstrip on the reef, which is also claimed by Vietnam, Del Rosario said: "That's one possibility."
Chinese Foreign Ministry spokeswoman Hua Chunying would not confirm the Philippine claims but said China had sovereignty over the reef.
"Whatever construction China carries out on the reef is a matter entirely within the scope of China's sovereignty. I don't know what particular intentions the Philippines has in caring so much about this," Hua said.
Philippine officials said the nation's military surveillance aircraft had been documenting large-scale earth-moving activity on the reef since January. Philippine aircraft searching for the missing Malaysia Airlines flight MH370 in March also spotted the reclamation, officials said.
Photos claiming to show reclamation around the reef have been circulated in the Chinese online community. The state-run Global Times, quoting an unnamed source, reported that China was merely renovating facilities on the reef.
Del Rosario said the Philippines raised the reclamation issue along with the deployment of Chinese coastguard ships at Second Thomas Shoal during a summit of the Association of Southeast Asian Nations over the weekend in Myanmar.
The protest marks the latest row between China and the Philippines in the South China Sea, after Manila decided to prosecute nine Chinese fishermen apprehended in the disputed Spratly Islands last week for poaching sea turtles. Two other crew members, both minors, were flown to Guangzhou on Tuesday.
A commentary by Xinhua said the prosecution of the fishermen was a provocation - no matter whether the allegation of poaching sea turtles was true or not - because China had indisputable sovereignty over the islands.
"All parties should also be reminded that ignorance of China's resolve to defend its sovereign land will induce consequences too severe for certain countries to bear," it said.
China and Vietnam have recently been engaged in a separate stand-off after Beijing deployed a mobile oil rig to the Paracel Islands, which China calls the Xisha Islands, and Vietnam calls the Hoang Sa Islands.
US Secretary of State John Kerry described China's move as "provocative" in a telephone conversation with his Chinese counterpart, Wang Yi , on Tuesday, US State Department spokesman Jen Psaki said.
But Hua denied Kerry had accused Beijing of being provocative, Xinhua reported.
Agence France-Presse, Associated Press