China accused the US on Monday of "hyping up" recent military flights around a Chinese island in the South China Sea.
It said Washington's motivation was questionable and urged countries concerned to show "restraint" over the territorial issue.
Observers said the US is apparently using the timing to expand military deployment in the area.
Washington's comments came ahead of a ruling by an international arbitration body in a process launched unilaterally by the Philippines against China's territorial claim in the South China Sea.
Asked about a South China Morning Post report on Monday that said China will start constructing an outpost on Huangyan Island this year, Foreign Ministry spokeswoman Hua Chunying said she had not heard of the case.
"However, I saw media reports that military planes from the US and the Philippines flew over the sea around Huangyan Island," Hua said.
She said China has always respected the right to normal and legal flights.
"But such high-profile hyping (of the flights concerned) is abnormal, and the motivation questionable."
She also said that Huangyan Island is China's inherent territory, and Beijing will "take necessary measures to protect its sovereignty and justified rights and interests".
Hua said China does not want to see further provocation by the countries concerned and hopes they will show restraint.
The Defense Ministry on Monday also voiced objection to the flights, saying they were being staged under the guise of navigation and flight freedom, but they were actually pushing forward militarization in the South China Sea.
The Philippines claims Huangyan Island, which belongs to and is controlled by China.
The Japan Times reported on Saturday that six US military aircraft left Clark Air Base in the Philippines on Tuesday last week and conducted "air and maritime situational awareness flights" near Huangyan Island.
The aircraft remained in the Philippines after a recent exercise by the two countries that included island-taking scenarios that were apparently targeted at China.
The US Pacific Command said in a statement on Friday that six military planes flew last Tuesday through international airspace near Huangyan Island.
US warships have stepped up operations around Chinese islands in the South China Sea, including one in October and another in January.
Teng Jianqun, a researcher at the China Institute of International Studies, said: "The US military wants to use the Philippine bases to monitor and threaten Chinese islands, including Huangyan Island. This shows that the US is updating its military deployment in the South China Sea."
Tao Wenzhao, a researcher of US studies at the Chinese Academy of Social Sciences, said the US hyping of the flights is "definitely related to the ruling by the arbitration body", which is expected within weeks.
During a Southeast Asian tour by Foreign Minister Wang Yi, which ended on Sunday, China agreed with Brunei, Cambodia and Laos that the South China Sea territorial dispute should not affect relations between China and the Association of Southeast Asian Nations.
The countries also called on nations outside the region to play a constructive role in the area.