In an interview with AFP, Delfin Lorenzana said he believed China would eventually reclaim the Scarborough Shoal, which sits just 230 kilometres from the main Philippine island of Luzon.
Beijing has already built up a number of islets and reefs in the South China Sea, installing military facilities on several of them.
Analysts say that similar installations on Scarborough Shoal could give China effective military control over the disputed waterway -- something the US has said it is not prepared to accept.
"If we allow them, they will build. That's very, very disturbing. Very much (more) disturbing than Fiery Cross because this is so close to us," Lorenzana added, referring to one of the Philippine-claimed reefs China has built on.
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Because of its position, another military outpost at Scarborough Shoal is seen as the last major physical step required to secure control of the sea.
An outpost at the shoal would also put Chinese fighter jets and missiles within easy striking distance of US forces stationed in the Philippines.
A UN-backed tribunal -- in a case brought by Manila under then-president Benigno Aquino -- ruled last year that the so-called "nine-dash-line" which underpins Beijing's claim to most of the South China Sea had no legal basis.
But his successor, Rodrigo Duterte, has courted Beijing and backed away from his country's close relationship with the United States.
Lorenzana said Chinese island-reclamation efforts were meant to control the South China Sea.
"That could be their strategy to counter any superpower that would encroach on South China Sea because they believe South China Sea is -- that's like their lake to them -- theirs," he added.
During confirmation hearings, Secretary of State Rex Tillerson said the US would block Chinese access to the islands, although analysts have pointed out that this would require a military blockade -- an act of war.
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