"Huangyan Island is part of China's territory," Foreign Ministry spokesman Hong Lei said during a news briefing.
Media reports said the Filipinos, led by a retired military officer, wanted to go to the South China Sea island and plant their country's flag before being barred from doing so by Philippine President Benigno Aquino III, China's state-run Xinhua news agency reported.
"We have taken note of the reports," Hong said. He said the Chinese will "remain on high alert over the island to prevent any provocative behavior."
"We also hope that the Philippine side will stop making irresponsible remarks and inciting radical behaviors," he said.
Hong urged Philippine leaders to "come back to the right track of a diplomatic solution" in the continuing dispute concerning sovereignty of the island and "send clear and consistent messages" on the matter.
Several countries have conflicting claims of oil and gas reserves in the South China Sea. China and Taiwan claim nearly all of the South China Sea while Brunei, Malaysia, the Philippines and Vietnam have claims on parts near their shores.
The dispute has occasionally flared into violence in the past four decades. In recent months, the Philippines and Vietnam complained of Chinese vessels encroaching on their territory.