"As we were conducting routine maritime air patrols and flying in international airspace, our air force aircraft were challenged over the radio," Vice Admiral Alexander Lopez told a Senate hearing in Manila on Thursday.
Lopez, commander of the Philippine Western Command, said the pilots ignored the warnings, replying that they are navigating international space.
While Lopez did not provide a timeframe, another senior Philippine air force official who asked to not be identified told Reuters that the six warnings had come in the past three months.
That official added that China could be "testing the waters" to see if it can enforce an air exclusion zone above the Spratly archipelago of the South China Sea, where multiple countries have overlapping territorial claims.
Recent satellite images show China has made rapid progress in reclaiming land around seven reefs it occupies in the Spratlys, including building what appears to be an airstrip on one of the artificial islands, Reuters reported.
"The Chinese said our planes were in their military security area," Vice Admiral Lopez told senators.
In late 2013, China imposed an Air Defense Identification Zone (ADIZ), in which aircraft are supposed to identify themselves to Chinese authorities, above the East China Sea. The United States and Japan condemned the move.
Chinese Foreign Ministry spokeswoman Hua Chunying said China had every right to set up ADIZs if it so wished. The situation in the South China Sea is stable, she added, and China and Southeast Asian countries want peace there.
"Under these conditions, I think that individuals hyping up an ADIZ, that China possibly wants to set one up in the South China Sea, this obviously has ulterior motives," she said.
On April 19, a Chinese warship challenged a Philippine air force plane that was approaching Subi Reef, which is part of the Spratlys. The pilot reported hearing the radio message: "Foreign airplane, you are approaching my military security area, please go away quickly in order to avoid misjudgment."
Lopez said China had expanded the seven reefs it occupies from a few thousand square meters to up to 11 hectares (27 acres) in artificial islands, including two areas close to the Philippine-held Thitu Island, Reuters reported.
China has denied accusations its actions are provocative, and even recently accused the Philippines, Vietnam and others of carrying out illegal building work in the South China Sea.
The US military commander for Asia, Admiral Samuel Locklear, said last month that China could eventually deploy radar and missile systems on its outposts that could be used to enforce an exclusion zone should it move to declare one.
Beijing claims most of the South China Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei also have overlapping claims.
