In a surprise announcement, Duterte told reporters he has ordered soldiers to build structures on Philippine-claimed islands and reefs in the strategically located Spratly archipelago.
"I have ordered the armed forces to occupy all," he said during a visit to a military camp on the western Philippine island of Palawan, the largest land mass close to the Spratly group.
"Erect structures there and raise the Philippine flag," he said, adding Manila was claiming "nine or 10" Spratly islands, reefs or cays.
China claims most of the sea, including waters and rocks close to the shores of neighbours and has been building artificial islands and installing weapons on them, including on some reefs in the Spratly chain that are also claimed by Manila.
"It looks like everybody is making a grab for the islands there, so we better live on those that are still vacant," Duterte said.
"At least, let us get what is ours now and make a strong point there that it is ours," he added.
Duterte also said he "may" visit the Philippine-claimed areas on June 12 to mark Philippine Independence Day.