Philippine President Rodrigo Duterte has warned of “unfriendly” treatment for foreign ships travelling in the country’s territorial waters without permission, in a rare swipe at China’s use of warships just a few miles off Manila’s coast.Duterte’s spokesman, Salvador Panelo, on Tuesday made the demand for transparency amid frustration by the Philippine military at multiple sightings this year of Chinese warships moving within the country’s 12 mile territorial sea, at various locations in the archipelago. “All foreign vessels passing our territorial waters must notify and get clearance from the proper government authority well in advance of the actual passage,” Panelo said.“Either we get a compliance in a friendly manner or we enforce it in an unfriendly manner,” he added. Panelo did not refer to China by name, nor elaborate on what that enforcement might entail.The Philippines has lodged several diplomatic protests in recent weeks over the activities of Chinese coastguard, navy and paramilitary fishing vessels in Philippine-controlled areas of the South China Sea and in its territorial waters. The armed forces has released images and cited witness sightings between February and early August of Chinese warships off Palawan and Tawi Tawi islands, a pattern that Defence Secretary Delfin Lorenzana last week described as an “irritant”.Duterte is facing heat at home for what critics say is his passive approach to Chinese provocations in exchange for a business relationship with Beijing that is not working out well for him, with promised investments slow in coming.Though surveys consistently show Duterte enjoying a level of domestic approval never seen at this point in a presidency, the same polls show growing disdain for China over its conduct in the South China Sea, and reservations among some Filipinos over a massive influx of Chinese online gaming workers under Duterte.Duterte will visit China from Aug. 28 to Sept. 2, his spokesman said. He has promised to discuss a South China Sea 2016 international arbitration victory over China with counterpart Xi Jinping.Duterte has until now chosen not to push that ruling, which invalidated China’s claim of sovereignty over most of the South China Sea. Beijing did not participate in the court proceedings and rejected the ruling.The South China Sea is a vital route for ships carrying more than $3 trillion in trade every year. The Philippines, Malaysia, Vietnam, Taiwan and Brunei also have overlapping claims to parts of it.