China has warned Philippine air force and navy planes at least six times to leave areas around the disputed South China Sea, the Philippine military commander responsible for the region told a Senate hearing on Thursday.
While Vice Admiral Alexander Lopez gave no timeframe, a senior Philippine air force official told Reuters the warnings had come in the past three months.
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, said the air force official, who declined to be identified.
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.
That has alarmed some countries in Southeast Asia and drawn criticism from Washington.
"As we were conducting routine maritime air patrols and flying in international airspace, our air force aircraft were challenged over the radio," Lopez, commander of the Philippine Western Command, told senators, adding the planes ignored the warnings.
"The Chinese said our planes were in their military security area."
China deploys coastguard and naval vessels in the Spratlys, but rarely planes because of the distance from mainland China.
China drew condemnation from Japan and the United States when it imposed an Air Defence Identification Zone (ADIZ), in which aircraft are supposed to identify themselves to Chinese authorities, above the East China Sea in late 2013.
Chinese Foreign Ministry spokeswoman Hua Chunying said that China had every right to set up ADIZs if it so wished, but that the situation in the South China Sea was stable at present and that 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.
Last month, a Chinese warship challenged a Philippine maritime patrol plane near Subi Reef, asking the low-flying aircraft to leave China's territory, said military spokesman Lieutenant Colonel Harold Cabunoc.
The U.S. 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 potentially energy-rich 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.
China has denied accusations its actions are provocative. It recently accused the Philippines, Vietnam and others of carrying out illegal building work in the South China Sea.
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, also known as Pagasa, and Second Thomas Shoal.
The United States and other countries would be welcome to use civilian facilities China is building in the Spratlys for search and rescue and weather forecasting "when conditions are right", China's navy chief told a senior U.S. officer recently.
