Huangyan Island is China's inherent territory, and the Chinese military will "take all of any necessary measures to safeguard state sovereignty and security", the Ministry of National Defense said on Monday.
Beijing made the response after media reports said six aircraft with the US Air Force embarked on a flying mission in the so-called "international airspace" near the island in the South China Sea on April 19.
"We have taken notice of the relevant report," the ministry said on Monday evening in a written reply to a media request.
The ministry said: "It needs to be pointed out that the US side, in the name of 'freedom of navigation and overflight', is actually advancing the militarization of the South China Sea."
Such actions "pose a threat to sovereignty and security of countries along the sea and sabotage regional peace and stability", the ministry said.
"We express concerns and opposition to this," the ministry added.