MANILA (DPA) - The Philippine government on Thursday (Aug 15) expressed concern over the passage of Chinese warships in the country's waters without prior coordination, saying it was not "an act of friendship".

Five Chinese warships have been spotted passing through a strait in the southern Philippines in separate incidents since July, the military said.

"We express concern with that kind of incident because they (China) keep on saying that we're friends," presidential spokesman Salvador Panelo said. "I don't think that this is an act of friendship."

"We will call their attention," he added, saying he will raise the matter when he meets China's ambassador to Manila for dinner "one of these days".

Philippine President Rodrigo Duterte is scheduled to visit China later this month, and plans to discuss the need for a code of conduct in the disputed South China Sea when he meets Chinese President Xi Jinping.

But Mr Panelo said it would be up to Mr Duterte whether he brings up the uncoordinated passage of the Chinese warships.

Lieutenant General Cirilito Sobejana, a regional military commander, said the Philippine military tried to contact the warships when they were spotted in the Sibutu Passage but the Chinese did not respond.

"We could not recognise where they came from or where they were going because their automatic identification system was off," he said.

"They entered our territory without any coordination."

China claims almost the entire South China Sea as its territory and has built artificial islands with military-capable facilities in the area. Vietnam, Malaysia, Brunei and Taiwan also have claims to the sea.