The Philippines expressed alarm Thursday over what it said were escalating Chinese efforts to drive off Filipino aircraft from a disputed South China Sea island garrisoned by Manila, in dangerous confrontations.

Rear Admiral Alexander Lopez said seven Filipino patrol planes on separate flights between Thitu island and Chinese-held Subi Reef in the Spratly Islands had been warned to stay away in radio messages from Chinese forces on Subi.

"Recently this area has been the source of air challenges to our aircraft landing and departing from Pagasa island," he told a hearing of the senate national defence committee, using the Filipino name for the Philippine-garrisoned Thitu.

The Philippine military last week reported an incident involving a Fokker plane which was challenged by a Chinese vessel on April 19. But Lopez, commander of Filipino forces in the South China Sea, said there had been six other warnings issued since then.

All seven Filipino aircraft were addressed as "foreigner planes", advised they were entering a Chinese "military area", and told to leave to avoid to avoid a possible "misjudgement", Lopez told reporters after the hearing.

"We are navigating in international airspace and conducting normal patrols," he quoted the Filipino pilots as replying. They did not alter their course.

"Fear will bring you no good... The risk is always there, but that's what we're being paid for," Lopez said.

China claims most of the resource-rich South China Sea, even reefs, shoals and cays close to the shores of its neighbours. The claims overlap those of Brunei, Malaysia, the Philippines, Vietnam and Taiwan.

The Spratlys are considered a potential Asian flashpoint, and claimant nations have expressed alarm as China has embarked on massive reclamation activity. Lopez said surveillance showed Beijing was enlarging seven features of the Spratly group that it occupies, including Subi.

Satellite photos last month showed a runway and harbour taking shape in one location which was little more than a reef when works began late last year.

The admiral said the reclamation would potentially give China air and naval bases in the disputed region and house "thousands" of personnel.

"These developments are disturbing to say the least, and alarming to say the most," Philippine Defence Secretary Voltaire Gazmin told the Senate hearing.

Asked if the Philippines feared China would eventually try to seize Thitu, Gazmin told AFP: "We have a problem but we haven't given up our claim to Pagasa. That remains ours."

"I don't think China is ready to go to war over small islands," he added.