The warning comes as Duterte faces critics at home who accuse him of being passive over Chinese provocations in exchange for business ties with Beijing, though promised investments have been slow in coming.
Duterte’s frustration over multiple sightings this year of Chinese warships moving within the country’s 12-mile territorial sea, at various locations in the archipelago, was relayed on Tuesday by his spokesman Salvador Panelo.
“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 in a statement, quoting Duterte.
“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.
Defense Secretary Delfin Lorenzana told reporters an unfriendly response could involve escorting the unwelcome vessel out of the area. “There are so many things that we can do to be unfriendly,” he said.
Opinion surveys consistently give Duterte 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 concerns among some Filipinos over an influx of Chinese online gaming workers under Duterte.
Duterte is scheduled to visit China next week where he has promised to discuss with counterpart Xi Jinping the Philippines’ 2016 international arbitration victory over China.
Duterte has chosen not to push that ruling, which invalidated China’s claim of sovereignty over most of the South China Sea, which is at odds with claims by Vietnam, Malaysia, Brunei, the Philippines and Taiwan.
Beijing did not participate in the arbitration and rejected the award.
Manila has lodged several diplomatic protests in recent weeks over the activities of Chinese coast guard, 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.
Lorenzana said the Chinese activity was concerning.
“The frequency has increased also, so it’s very alarming why so many navy ships of the Chinese are passing through,” he said.