A Philippine air force jet is conducting daily patrols over Chinese fishing vessels parked near a disputed reef, the defence chief said, as Beijing refuses to pull the ships out of the area.
Around 220 boats were first spotted earlier this month at the boomerang-shaped Whitsun Reef around 320 kilometres (175 nautical miles) west of Palawan Island, sparking a diplomatic row.
Manila has ordered Beijing to recall the vessels, describing their presence as an incursion into its sovereign territory.
But China, which claims almost the entirety of the South China Sea, says the flotilla is made up of fishing vessels sheltering from bad weather.
The Philippine foreign ministry has filed a diplomatic protest, and several countries, including the United States and Australia, have expressed concern over the renewed tension in the region.
Philippine navy and coast guard ships have been deployed to the area to monitor the situation, in addition to the aerial patrols, Defence Secretary Delfin Lorenzana said late Saturday.
"We are ready to defend our national sovereignty and protect the marine resources of the Philippines," Lorenzana said, repeating a call for the Chinese ships to withdraw.
He added there will be an "increased presence" of navy and coast guard ships patrolling Philippine waters.
The resource-rich South China Sea is contested by several countries, including the Philippines and China.
Beijing often invokes the so-called nine-dash line to justify its apparent historic rights over most of it, and has ignored a 2016 international tribunal decision that declared this assertion as without basis.
President Rodrigo Duterte has expressed concern over the presence of the vessels to the Chinese ambassador in Manila, presidential spokesman Harry Roque said Thursday.
Duterte has fostered warmer ties with Beijing since taking office in 2016 in exchange for greater economic cooperation with its superpower neighbour.
But the shift has failed to stem Chinese ambitions in the South China Sea, or unlock much of the billions of dollars of promised trade and loans.